Signal System Design

[rr503]

I've been working a lot with signals on the rapid transit side of things lately, and in my free time, am trying to learn how things work over in freight. I have a few questions.

We all know that signal spacing is determined by stopping distance. On the RT end of things, train length/weight basically don't matter because each car has its own braking apparatus, but on freights this isn't the case, which then suggests that there must be some length/tonnage limit for trains transiting a given line. That much makes sense to me, but I guess my question comes in areas with old signals today handling 10k+ freights -- how does that work? Do DPUs become mandatory?

These are disorganized questions; all answers are appreciated. Happy new year!

[Wayside]

Freight cars are all equipped with braking systems, as mandated first by the Railroad Safety Appliance Act, followed by several legislative and regulatory updates over the years: https://en.wikipedia.org/wiki/Railroad_ ... liance_Act

Freight trains over a certain tonnage are usually restricted in speed (i.e.: 40mph), since speed is the largest determining factor in stopping distance, followed by tonnage.

[rr503]

Freight cars are all equipped with braking systems, as mandated first by the Railroad Safety Appliance Act, followed by several legislative and regulatory updates over the years: https://en.wikipedia.org/wiki/Railroad_ ... liance_Act

Freight trains over a certain tonnage are usually restricted in speed (i.e.: 40mph), since speed is the largest determining factor in stopping distance, followed by tonnage.

I am aware of air brake systems lol. But a locomotive can only pressurize a certain amount of line, no? And braking efficacy will vary with a car's load (solved by variable load gauges in my part of the world -- do equivalents exist?). And then you have length-dependent variations in trainline response time. I guess my question is whether or not the only way these changes are accounted for just a lowered train-specific speed limit.

[Wayside]

Max speed is usually determined by a tons per operative brake calculation. Railroads increase stop preparedness by using advance approach signals. And yes, using DP improves brake pipe charging and brake control for longer trains.

[rr503]

Gotcha. This is what I was looking for.

So they calculate the stopping distances that underpin signal spacing from the flashing yellow--ie giving two blocks to stop before the red?

[Wayside]

So they calculate the stopping distances that underpin signal spacing from the flashing yellow--ie giving two blocks to stop before the red?

Correct.

[Engineer Spike]

Sometimes it’s not just getting a flashing yellow. In some places the signals are placed really close together. Where I work we have speed signaling. Sometimes the signal sequence will get you down to medium, or even slow speed before the approach signal. The last signal befor the stop could also be a slow speed signal.

My whole point is that beside the signal spacing and tonnage restrictions, the signal system has many more indications than just go, caution, and stop like a highway traffic signal. The previous poster was correct in the fact that with a train not being able to stop within the length of vision, the signal system works the train down to safe speeds for the conditions. This doesn’t mean that sometimes the engineer doesn’t have to be more cautious. In the winter the snow sometimes lubricates the wheels against the normal brake friction. With the engineer being required to be familiar with each line he runs, he may proceed more carefully than he normally would.