If you have at least the standard license you can save the game and send the saved file over to me. Without that or any other detail it is difficult to say what is happening.
This jumping back and force is designed to happen if a collision is imminent, and it is there to avoid to have to untangle the mess (software wise) when it comes to remove the out-of-service trains later. The speed is set to 0 right away as a result of the collision - and of course you'll notice that when that happens.
But this seems to be not the case here. The distance you see in the train details window is the distance to the end of the current block, or the point where the head of the train is supposed to stop at the next station stop, whatever comes first. It gets smaller as the train moves long, until the end of block is reached, or a stop is honored. With 4100 ff (and no train ahead of it) there is no reason for anything else than running smoothly.
I have not encountered or heard of a similar situation like you're reporting. So I'm interested in more details. As said above, a saved file is the best way to start. If you run only the free version, you cannot save the game status, in which case I'd like to get at least the block number where the train head is located. E.g. you can make a screen snapshot showing the detailed train information
