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This might not even apply these days. I was reading an old GTW Timetable from the late 60s. In the back, in the general speed restrictions area, it listed 30mph limit for a single diesel running light. But two or more diesels running light were allowed 50mph limit. Why are two or more diesels running light, allowed a higher speed limit. Or does that even apply these days. Does a single loco running light, have special characteristics that would limit it's safe speed?

Still applies today. A single engine by itself can lose shunt at higher speeds, meaning it might not trip signals or activate crossing protection, because it doesn't have enough axles. 2 or more units eliminate this problem. Even an engine and a single car wouldn't be restricted, either.

Has to do more with braking than anything else. More wheels, faster braking, more control.

SD80MAC wrote:Still applies today. A single engine by itself can lose shunt at higher speeds, meaning it might not trip signals or activate crossing protection, because it doesn't have enough axles. 2 or more units eliminate this problem. Even an engine and a single car wouldn't be restricted, either.

Not sure where you are getting your info, or if other railroads are different, but light engine(s) on UP are allowed 70mph as long as the dynamic brakes are sufficient enough to control the speed. If the dynos are not sufficient enough to control speed, the engine(s) are limited to 45mph. So, if the unit doesn't have dynos, or the dynos don't work for crap, you're limited to 45. Back in the day, most units didn't have dynamic brakes, so if you only had one unit and something happens to the air brake system, you're out of luck. And as Matt said, you have less brakes when you only have one unit. If you had two or three units and the brakes fail on the lead unit, you still have one or two units back there to stop you.

Interesting two perspectives. I have heard from multiple CN employees that the reason CN (GTW) restricts lite engines is due to signaling issues as SD80MAC explained.

bnsfben wrote:Interesting two perspectives. I have heard from multiple CN employees that the reason CN (GTW) restricts lite engines is due to signaling issues as SD80MAC explained.

I've heard both, but to add to the signal issues, you aren't supposed to stop on sand with a single light unit to avoid a problem of not shunting the signal system.

Practice Safe CSX

M.D.Bentley wrote:Has to do more with braking than anything else. More wheels, faster braking, more control.

"faster braking"? "more control"?

M.D. Bentley....Have you ever run such a train before? Are you an engineer? You are using faulty logic; your statement holds absolutely no merit, and here's why:

1) The brakes/tons ratio (braking axles per ton) is the same for two locomotives as it is for one unit. Therfore, if we apply your sense of logic, the restriction would still be necessary for 2 or more locos. And one boxcar (or any other rolling stock, for that matter) provides hardly any control or braking effort on a one-unit, one-car train, unless it's empty and you go right down to 70 lbs on your first application. And even then, it takes a while to have any affect at all. Not much control there. And as for the "faster braking", I've never heard an engineer actually use those words together (but I have heard "faster release", something usually associated with warmer weather vs. freezing temperatures)

2) What SD80MAC said was spot-on. That was the exact explanation given to us at engine service school, and it is a CSX rule still today. (I cannot speak about other carriers, but I can verify that is what they say of the rule at CSX).

Mr. Tops wrote:....so if you only had one unit and something happens to the air brake system, you're out of luck. And as Matt said, you have less brakes when you only have one unit. If you had two or three units and the brakes fail on the lead unit, you still have one or two units back there to stop you.

As to what Mr. Tops stated, I wonder how, if the lead unit air brakes failed enroute, that the others would not; as the trailing units are cut-out and tied directly into the lead units' air brake control system, they too would most likely not apply! And a failure in the air brake system on any unit in the consist causes it to go into emergency. (I actually had an air brake system failure on the second of 3 units, and it put the train into emergency, as it is programmed to do during such a failure. This happened 6 times enroute, plus the 2 times it happened to the previous engineer!)

Just backing up what a few others said here, namely SD80MAC, from an engineers' perspective.

-barny