Hello Mr Inn,
Either you have placed the wrong type of speed signs on your route, or your signals are not placed close enough for the AI to understand them, or you have a break in the track.
If you have placed the killometer per hour (kph) sign and not the miles per hour (mph) sign, then you might get a different speed to what is shown on the speed board, have a look for UK Speed signs, there are some with left hand and right hand placing, placing a UK 70 MPH speed sign, should then set your AI train to do 68 - 70MPH. You can use the replace tool, if you have too many speed signs on a huge route.
If you have already done the above, and you still get a train that crawls, then it is either because it is not well signalled, placing signals about a board in length apart, usually corrects this, or there is breakages along the route that the AI knows about, but doesn't know where.
Placing signals along the track, will eventually tell you where the breaks are, if you see a red signal on a clear stretch of line, followed by a green, then the break is between the red signal and the green signal.
Hope the above helps and please let us know how you get on.
Joe Airtime