I'm not aware of this being possible with the AI trains but it can be done with the SCS2006 rule. Within this rule are a number of commands some of which allow each train to have its maximum speed set. An express can have its maximum speed set to 100 mph for example while a freight train on the same track can be set to a max of 40 mph. They won't exceed their max speed irrespective of any speedboards on the line. A train can also have its maximum speed varied as it travels along the line.
The rule has a very steep learning curve but I have found the time spent learning it very worthwhile. It's on the DLS if you want to have a look at it but make sure you download the manual as well.
I have not used these myself, but try a Content Manager search for "VSB" (Variable Speed Board) and "Driver Speed Rule" - various authors and countries. Some are built in and some on the DLS.