Western Railroad Discussion > Vacuum Cleaner

Here's the ISEG4 with an SD70AH Tier 4 leader at CP W350 on the Sidney Sub, just east of Big Springs (NE).  Although these Tier 4 units look like a regular ACe from the front, they sound like nothing more than a big vacuum cleaner.  The horns sound different, too.

mike woodruff
north platte ne

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Most excellent! Good looking train and all the colors in the frame is really something.
Brian Stevens

Posted from iPhone

Neat catch and shot!

swaool Wrote:
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> Although these Tier 4 units look like a regular ACe
> from the front, they sound like nothing more than
> a big vacuum cleaner.

Nice catch and nice shot!

GE's Tier 4 ET44AH also sounds vacuum cleaner-ish. but the SD70AH-T4's "whoooooooosh" seems to stand out more.

SD70ACes use 2 stroke prime movers, hence its "whirrring" sound while SD70AH-T4 use 4 strokes like GE's, If the exhaust wasn't so quiet, T4 EMDs would have similar "thumps" to the GEs. Those who have compared the exhaust noises between dirt bikes with 2 strokes and road bikes with 4 strokes should understand the source of the sound difference.

And it takes more than turbochargers (GP9 vs GP20 or GP38 vs GP40 sound difference) and emissions equipment (Tier 4) to make the sound so much quieter than Tier 2 and Tier 3 engines. I suspect there may be tuned pipes (measured to specific sound frequencies) and baffolds inside mufflers (mechanical version of noise-cancelling earphones) to make T4 engines sound like vacuum cleaners.

I used to experiment with engines for model aircraft (methanol as fuel) to reduce noise while preserving as much horsepower as possible. Most gearheads (car guys) prefer more exhaust noise, but sportscar engineers have mastered the art and engineering behind the exhaust note, so they generate emotionally satisfying signature sounds.

> The horns sound different, too.

All of UP's SD70AH-T4s I've caught so far have the K5LA, quieter and less dissonant version of SD70ACe's "wake the dead" K5LLA. K5LAs are common on Amtraks, I hear them occasionally on NS and CSX locomotives.

cchan006 Wrote:
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> All of UP's SD70AH-T4s I've caught so far have the
> K5LA, quieter and less dissonant version of
> SD70ACe's "wake the dead" K5LLA. K5LAs are common
> on Amtraks, I hear them occasionally on NS and CSX
> locomotives.

Almost all NS and CSX units have some variant of the K5LA on their general roster. They are really nice sounding horns and make for a great surprise when youre so used to the commonality of UP and BNSF's K3LAs lol 

Back in 2025, I caught an eastbound UP climbing the hill at Shawmut, AZ with UP 3018 as a rear pusher on a long manifest, who was working very hard as one of the lead units had died. I wish I had recorded a video; but I was surprised how quiet it was compared with a 710 engine. It still sounded good, but not the gutsy, howling sound that makes 710 sounds so much better.

Tom

Great shot!

Very nice..!!