Western Railroad Discussion > Can you see through Mt Macdonald Tunnel?

The Trains article says the CP's Mt Macdonald tunnel is 48304 ft long and 25 ft 10 inches high. If it's a constant grade (is it?) that probably means a person standing in the middle of the track at one portal can't see any light at the other end, since the tunnel follows the curve of the earth. Has anyone ever tried?

No, you can't see through it. You can, however, hear a train a good 20 minutes before it gets to you.

Yes I have stood at the west portal (which is the summit). No you can't see all the way through. An approaching train rises from the floor due to the curve of the earth effect. Loaded westbound trains take 25-30 minutes to pass through the tunnel and the sound and anticipation goes on forever.

cp1400

Aside from the earth curvature, the portal subtends only about half a mil from the other end. Your eye only resolves about one mil (one foot wide at 1,000 foot range) so the "light" would have to be very bright to be seen by a human unaided eye.

Lindsay

We'll allow binoculars. But you're saying if you looked through a 100-foot pipe, half an inch in diameter, you couldn't see the other end, with the naked eye?

Been through it during clear afternoon sunlight when the west portal would be well lit, and there was NO light at the end of the tunnel.

Why would a tunnel necessarily follow the curvature of the earth? Wouldn't it behave like a mathematical chord?

I don't know about this particular tunnel, however I can't say I know of any "long" tunnels that are actually straight. Given that they are drilled from the ends toward the center, the working face is driven slightly uphill so that water drains away from it during construction. Thus there is an "apex" in the center at the intended meeting point of the two working faces, and you can't see past the vertical bend at the apex when looking in from either end. The only long tunnel I can remember that drilled downhill was the east end of the Moffat, and that was only because the west end crew ran into so much bad ground that the east end crew got way ahead of them, arrived at the intended "apex" first, and then rolled over to keep going. After that, the east end crew ended up with completely flooded workings a couple of times after passing the apex when they hit pockets of high pressure ground water, thus confirming the wisdom of drilling uphill.

On very long tunnels, earth curvature can prevent you from seeing the ends from the apex. I think it was the Simplon Tunnel in Switzerland/Italy that used a lamp at the south tunnel portal as a target for back-sighting by surveyors as a means of keeping the tunnel straight. After a while the started noticing the light was higher above ground than it should have been as seen from the working face, even though a level line showed the tunnel was being bored straight. This puzzled them until they figured out it was due to the curvature of the earth. That was the first instance of this effect in engineering history.

- Kit

The only long tunnel I've looked through is the Cascade Tunnel on Stevens

Seems you also forgot this tunnel isn't straight (Kit is sort of right) there's a slight bend at the east end. Yes I've looked into both ends and no, you can't see through. Estimate it would take 12 minutes to ride a bicycle through, it is concrete-lined all the way through and no ties.


TCS - engraved

The curvature of the earth thing, the great circle, applies to the surface of the planet and the water parts in particular. When you drill or dig a tunnel there is no need to follow the surface, you can dig a chord as in a perfectly straight line anywhere you want to do it. It would be far more complex and engineering problem to follow the earth's curvature when setting the cut than to do a truely straight line. I question there is any vertical curvature in this tunnel or any other due to the curve in the surface of the earth.

I can see this going over 20 posts in a hurry right away. I have made my point and have nothing more to offer. Correct me if you can.

BCHellman Wrote:
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> Why would a tunnel necessarily follow the
> curvature of the earth? Wouldn't it behave like a
> mathematical chord?

Agree. Especially since this tunnel was drilled using hi-tech surveying techniques.

As other posters said, few "straight bore" tunnels are perfectly straight. Depending on length, some tunnels are bowed in the center to facilitate drainage. Some tunnels at crests of grades are also bowed in the middle for operational reasons.

Even in a theoretically straight tunnel, daylight from the other end would be imperceptably dim after some distance. Bob

You need to distinguish between "straight" and "level" in surveying. Pull out the old surveying text if you have one and look up how you lay out a "level" grade. It is done by setting up the level transit, taking a backsight on where you've been, and then a foreward sight on where you are going. The accuracy depends on how well you leveled the transit before taking the sightings. Since you are working around a globe, and "level" is always the perpendicular to the radius line to the center of the globe from where you stand, "level" won't make a straight line but will bend with the surface of the globe as you move around it. So if you use a level to control direction of tunneling, your tunnel will curve in a vertical plane as you go, just as laying out a "level" tangent across Kansas will bend as you head from Colorado toward Missouri despite the legendary (and inaccurately stated) flatness of the state.

It isn't easy to lay out a truely straight line. At the risk of getting in even more over my head, I'll add the following: The best way to lay out a "straight" line is to depend on line-of-sight to some target at the other end of the "straight" section. But even that gets distorted by varying air density in the atmosphere, and by gravitational bending if you let the distances become great enough. So even a ray of light won't take a perfectly straight path.

Ok, now I'm seriously in over my head. Where's the beer?

- k

CimaScrambler Wrote:
-------------------------------------------------------
> You need to distinguish between "straight" and
> "level" in surveying. Pull out the old surveying
> text if you have one and look up how you lay out a
> "level" grade. It is done by setting up the level
> transit, taking a backsight on where you've been,
> and then a foreward sight on where you are going.
> The accuracy depends on how well you leveled the
> transit before taking the sightings. Since you
> are working around a globe, and "level" is always
> the perpendicular to the radius line to the center
> of the globe from where you stand, "level" won't
> make a straight line but will bend with the
> surface of the globe as you move around it. So if
> you use a level to control direction of tunneling,
> your tunnel will curve in a vertical plane as you
> go, just as laying out a "level" tangent across
> Kansas will bend as you head from Colorado toward
> Missouri despite the legendary (and inaccurately
> stated) flatness of the state.
>
> It isn't easy to lay out a truely straight line.
> At the risk of getting in even more over my head,
> I'll add the following: The best way to lay out a
> "straight" line is to depend on line-of-sight to
> some target at the other end of the "straight"
> section. But even that gets distorted by varying
> air density in the atmosphere, and by
> gravitational bending if you let the distances
> become great enough. So even a ray of light won't
> take a perfectly straight path.
>
> Ok, now I'm seriously in over my head. Where's
> the beer?
>

Good explanation - you've earned that beer. Bob

It seems to me that.

1. If a RR runs along a lakeshore or along the coast it is flat as far as gravity is concerned. It takes the same amount of power to pull a train in either direction and a train that is stopped on the track will not roll. But the track is not "straight" in the vertical plane. It is humped as it follows the curve of the earth.

2. If you build a say, 70 miles long, concrete tunnel liner over this track and perhaps pile 10,000 feet of dirt and rock on top of it to form a mountain, you will not be able to see the light at the end of the tunnel because of the vertical hump.

3. If you bore a new tunnel from portal to portal in an exact straight line both vertically and horizontally, say using laser sights, with no vertical curve then the tunnel will be shorter than the original. And it will appear flat as you look thru through it and you can shine a light through it. The tunnel would form a chord of the curve of the earth's surface.

4. What happens to the train running thru this straight "flat" tunnel? It seems to me that it would roll downhill to the center then have to be pulled uphill to the far portal?? Wierd. Water should run to the center of the tunnel and fill it up :-)

Even the most "modern" survey requires a control network. In the case of a non-GPS survey (which a tunnel would be because you can't use GPS underground), a tight traverse would be needed. These are based on visibility and normally don't go more than a half mile between control points. A multi-mile tunnel would require several setups--all based on leveling up the "gun"--with level based on the center of the earth. ALL large-scale pgojects (long highway or rail projects) require a control network and ALL have slight error based on curvature. If you were to take a section ofthe earth and make it :flat" by shaving it so, you would actually be creating a round lake. A multi-mile, laser-straight tangent would actually be a sag.

I'm not going to get into the engineering stuff, but I can confirm that waiting at the West Portal, I could hear a train coming long before I could see any evidence of it -- not even headlight reflecting off the tunnel walls.

"I can't say I know of any "long" tunnels that are actually straight [constant grade, I think he means]."

Cascade isn't?

As Mr Krug said, a tunnel that followed a chord (an actual straight line) between two points at the same elevation would be downhill from each portal-- 0.1% if the tunnel is nine miles long.

Drainage during construction sounds like a legitimate concern, and I sure don't have another explanation for Moffat's profile. But Mt Macdonald tunnel is uphill all the way to the west end? So quite likely a constant grade?

I've been through the Table Tunnel on the Tumbler Ridge (Tumbler Sub) line to Tumbler Ridge, BC four times. As I remember it is something like 5 3/4 or 6 miles long. (just under 6 miles) "To me" the "the light at the end of the tunnel" went away maybe four miles in. Others standing beside me claimed they could see it quite a bit longer. Nobody could see it all the way. I couldn't tell that there was any change in grade when I lost sight, a little pin prick of light just disappeared. My video doesn't show it five miles in.

jb

AAK Wrote:
-------------------------------------------------------
> It seems to me that.
>
> 4. What happens to the train running thru this
> straight "flat" tunnel? It seems to me that it
> would roll downhill to the center then have to be
> pulled uphill to the far portal?? Wierd. Water
> should run to the center of the tunnel and fill it
> up :-)
>

This is true. Now for the physics majors out there: Assuming that there is no friction and no air drag, a car released at one end of a geometrically straight or chordal tunnel would accelerate to the middle and decelerate to the other end, reaching the other end at zero velocity. How long will it take the car to reach the other end? Interestingly, the time does not depend on the length of the tunnel. However, it does depend on the density of the earth.

I will post the answer later.