F-units, freight, and speed limits

Model Railroader Prototype information for the modeler
1

I checked The Diesel Shop website and it states that the top speed for a 4-axle F-unit was 65 MPH. What was generally the top speed when pulling freight and no speed restrictions? Did most engineers top it out in those circumstances?

I’m just trying to get an idea of what a “fast” prototypical speed would normally be when operating an FT, F2, or F3 and pulling freight on a single or double track mainline.

Thanks,

Tom

2

There are three components to the maximum allowable speed:

  1. The max speed of the engine - how fast the locomotive is allowed to go.
  2. The max speed of the train - how fast the railroad will allow a certain type of train to go, usually depends on the types of cars.
  3. The max speed of the track - how fast the quality of the track is and the alignment of the track (curvature and grade).
    For a freight F unit 65 is the typical max speed. An expedited freight train, auto parts, intermodal or perishable might be allowed to go somewhere around 60-65 mph. A general freight train somewhere around 50-60 mph. If it has a scale test car then 25 mph, MofW cars, maybe 40 mph. Lastly is the type of track. If the track has cab signals and automatic train stop then it can go over 79 mph. If it has block signal systems (but no cab signals) then trains can go up to 79 mph. If it doesn’t have a block signal system then freight is limited to 49 mph and passenger to 59 mph. If the track is on a curve the spped will be lower.

Assuming that the train is on straight and level track, with a block signal system and not over tonnage, then a fast freight train, with no speed restricted cars and no slow orders, would probably be between 50 and 60 mph (and that could be the same answer whether its an FT or SD70AC).

If its in dark territory then it doesn’t matter what type or engine or freight train, its restricted to 49 mph.

3

Thanks, Dave! - Great info!

Tom

4

Hello, Tom,

An employee timetable is your friend:

NYC_TT12 by Edmund, on Flickr

This is from New York central Timetable No. 12, Sunday Oct. 28, 1962.

NYC_TT12_0001 by Edmund, on Flickr

There’s about five more pages of specific restrictions for particular equipment on specific track locations. NYC, of course had the two A-B-A sets of passenger F3s which were geared for 100 but your primary focus is on freight.

NYC_DCA-1_3500 by Edmund, on Flickr

I can probably dig up a few older timetables if you’d like. As I recall your era was a little before '62.

Good Luck, Ed

5

The Speed of any unit is dependent on the gearing of the unit and as I understand it, units could be bought with different gear ratios for speeds something like 62, 89 and 102 mph.

The higher the max speed, the lower the starting tractive effort of the unit so F units would have been geared appropriately for their intended service by the ordering railroad.

The other statements re track, signalling, load etc are also correct.

Here in Australia, Alco units similar to FA’s could outpull GM units similar to F3’s and F7’s because the gearing and traction motor rotationof the Alco units was good to 71 mph. The GM units were almost universally geared to 89 at max input on the three systems that ran them.

I would think a similar equation could be applied to North American units.

For what it is worth,

Regards

Trevor

6

Early-to-mid 40s, Ed. But I’ve opened my window a bit now to include up to early 50s.

7

Here is a few pages from the April 25, 1948 Cleveland Division timetable that is a little closer to your era:

NYCS_TT63_0002 by Edmund, on Flickr

NYCS_TT63 by Edmund, on Flickr

NYCS_TT63_0001 by Edmund, on Flickr

Employee timetables are a great resource for model railroad operations when you consider all the operating variables encountered.

NYCS_TT63_0003 by Edmund, on Flickr

Hummm, now you got me curious. Why were two L-2d Mohawks singled out that could run at 70 MPH with passenger trains while other L-1 and L-2s were limited to 60? Roller bearings, maybe? I’ll have to do some digging.

Hope that helps, Ed

8

The gear ratio decided the nominal top speed of the F-unit, and as others stated, track conditions and train type controlled the top speed of the train. The gear ratio could be changed, and CPR did that for most of their FP7As, changing the top speed from 65mph to 89mph, and later returning many of those back to 65mph gearing as passenger trains dwindled. There are other gear ratios available so it will depend on the particular prototype’s preference. For freight use, 65mph is very widespread.

Various gearing options were available for diesel models from all the major builders.

John

9

Tom,There is another factor that needs to be mention. If the track speed as (say) 40 mph the majority of the engineers I worked with would stay on 35mph this gave them a margin of error of 5 mph in case the speedometer was off and they would need less braking going down a slight grade.

10

The top speed is determined by the maximum rotational speed of the traction motors. As it spins faster under heavy load, the copper winding heat up, become soft, and move outward due to the centrifugal force. When they touch the outer case - Ground Short with DC traction motors. A high speed gearing will reach that maximum rotational motor speed faster.

11

Not to nitpick, but the massive ground short is only part of the fun with birdsnesting – by the time the epoxy insulation potting the windings has worn away on the pole faces, there’s a world of pain and probably locked-rotor surprise for the hapless or unwary.

And you mean low-speed gearing reaches that maximum rotational motor speed faster. I know you know that, but someone asking this kind of question probably does not.

A key point about ‘top speed’ that isn’t immediately obvious involves running the unit over grade crossings and crossovers, where there may be a ‘short sharp shock’ that bangs the brushes, levitates the growing pool of carbon worn off the brushes in service, and produces the dreaded prompt flashover if the motors ‘see’ high current at that point. From as early as 1945 there are warnings in the EMD manuals to decrease throttle notch before crossing such obstacles…