The problem is that the train won't sense a speed limit until it passes a speedboard. You, quite reasonably, want to set a speed limit which will apply until the train passes the first speedboard.
This is possible in a scripted scenario. The GameScript coding to do it is very brief but obscure to put it mildly. One of my RBR colleagues discovered or invented it, clever man. I'd better not include the coding here as I suspect that it might be subject to an RBR NDA.
I don't know if anyone's put that coding into a Rule for use in a Driver session. I guess it would be possible. I just had a quick search on DLS but couldn't find anything.
With enough programming skill it's probably also possible to add coding to search backwards along the track for the relevant speedboard and automate the whole shebang. However, there would still be a need for manual specification when you start by heading out of a dead-end spur.
John