What I have read/been told is the above is the most common reason for surging down hill: play in the gears and or motor. Too much and you get the effects mentioned. Maybe not such a big deal now with better quality motors and gear boxes but still could be a strong contributor.
A 1% grade will effectively halve the length of train that a locomotive can handle, going upgrade. If you introduce a curve into the same equation, expect a further reduction in train length, depending on the severity of the curve. Here’s a link to a very interesting and useful site:
As for the downhill surging, it seems as if the advent of DCC has obviated the use of the throttle as the primary means of speed control (I’ve noticed that some modellers seem to “re-discover” that a throttle adjustment will slow the train when it’s running downhill)[;)] First off, the prototype usually has a speed limit on grades: if you’re running fast enough on a 2.5% down grade that the slack is stretched, you’re going too fast. The prototype can stretch the slack, even at low speeds, because the brakes on the cars can be applied independently of those on the loco, a luxury that our models don’t have. So, slow down a bit: the train should be under control at all times, and this is a good way to learn proper train handling. You need to match the loco’s speed to that at which the cars will roll if there was no loco impeding things, while still keeping within the speed limit. Without a speed limit, you could simply increase the speed steadily to prevent the slack from running in, and obtain no surging, although it would look pretty silly. Another cause of surging is excessive gear lash, exacerbated by end-play in the locomotive’s motor. When the train is moving uphill, the g