Steam & Excursion > Tractive Effort of ATSF 2900 class 4-8-4 locomotives.

Santa Fe rated all of its 4-8-4 locomotives as producing 66,000 lbs of tractive effort. I question that same figure for the 2900 class 4-8-4 locos since they weighed more than the other classes of ATSF 4-8-4s. Has there been a newer calculation of the tractive effort of the 2900 class ?  Thanks in advance. 



Edited 1 time(s). Last edit at 06/28/26 20:59 by UP951West.

2900-2929 tractive effort was 79,968 lbs. according to everything I ever read about them.

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Chris Bost
Leesport, PA



Edited 1 time(s). Last edit at 06/28/26 21:26 by RBMN-ENGR.

Remember, starting tractive effort is only a factor of steam pressure, cylinder size and driver size.   Weight only enters into the factor of adhesion, tno T.E.

Well, to be fair, you have to look at both.

Your 'classic' steam tractive effort is a function of boiler pressure, driver size, cylinder size, stroke, etc.   

The 'practical' is simply weight on drivers off a clearance diagram and take 25% of it as a steel adhesion factor.   What's amazing is how close the two can actually be in a proper design.

Get too much BP, and not enough driver weight, and she's slippery by nature. bit the top-end speed and horsepower is possibly higher.   The 2900's had 300PSI.

If you like playing these performance calculations out here's my go-to source:  https://www.steamlocomotive.com/locobase.php?country=USA&wheel=4-8-4&railroad=atsf#236

And of course that has nothing to do with horsepower curves.

But in the 'whose engine was better?' debates that still rage, that's some hard numbers to consider.



Edited 1 time(s). Last edit at 06/29/26 08:15 by randgust.

The usual formula for "tractive effort" of a two-cyl engine is

cylinder diameter (inches) squared
times stroke (inches)
times boiler pressure (psi)
divided by driver diameter (inches)
times 0.85.

So for 3765 or 3776 or 2900, the usual
formula gives 784 x 32 x 300 divided
by 80 times 0.85, which is 79968 lb.

Which is wrong, because the only reason
the engine has 28-inch cylinders is to make
up for its 60% limited cutoff. In other words,
the 0.85 factor (which is reasonable for
most engines) is too high for a 2900.
That's why SFe only claimed 66000 lb TE.

Notice that N&W only claimed 80000 lb
TE for a 300-psi 4-8-4 -- the usual calculation,
with 0.85, gives 84980 lb.



Edited 1 time(s). Last edit at 06/30/26 12:38 by timz.

Somewhere the test results have been posted about the tractive effort of the 2900's. From what I have been told, their tonnage ratings were continually revised upward, which might lean intot he higher tractive effort. Also, ATSF Mechanical might have deliberately set those tractive efforts at 66K, while Operating found out the 2900's were more stout than originally thought. Santa Fe did that with thier 2-10-4's, which tested out to over 100k while their published tracting effort was 93k.

The 2-10-4s were 67% cutoff, so SFe made a reasonable
guess at their TE. (Wonder if they could sustain 100000 lb
on a halfway-long climb.)