I suspect that it means the last truck slipped off the track as it approached the siding.
A siding (not a "sliding") is a stretch of track set to the side of the main track where trains can pull off to let another train pass. There is a switch at the beginning and the end of a siding. It sounds like the last truck on the last car failed to negotiate the switch properly.
In this image the mainline track is to the left and the sidings are to the right:
Actually, it was in the context of a physics question.
A goods train travelling at 18m/s slips the last truck as it approaches a siding. The truck, which is retarded (decelerating) uniformly, travels 117m before it comes to rest in the siding. If the train does not alter its speed, how far will it be from the siding when the truck comes to rest?
And no, there is no past after the slips...it would make more sense if there was a 'past', wouldn't it...
I don't know whether the context provided helps a great deal, after all the rest of the question doesn't really seem to be that clear either.
It might be that the truck is being used here to describe the last railcar. A flatbed car is sometimes called a truck. I guess it doesn't really matter.
Something, a car or a truck assembly (most likely a railcar), is detached from the train at a siding. The truck slows down and comes to rest on the siding. If the train did not slow down, how far did it travel from the point where the siding started?
In other words, how far could a train traveling at 18m/s travel in the same time that it takes a body to travel 117m when it is decelerating from 18m/s to a complete stop. I suppose the first thing you would need to do is to figure out how long it takes for something to decelerate from 18m/s to a complete stop in 117m.
Railmen can figure the natural deceleration of a car so they can disconnect the car as the train passes the siding, flip the switch after the train has passed and catch the loose car so that it rolls onto the siding. This saves the trouble of coming back with another engine to push it onto the siding.