Thats an interesting problem, I tend to find the exact opposite to be the case on about every other route, like having a 79mph speed limit thru a sharp S-turn or something like that. I do have a possible solution for you though. In most cases the speed limit is determined by the kind of service your train is listed as, express passenger, express freight, ect. Now I find the fastest speed limits are for trains listed as a special service. To change your train to a special service if you don't know how is easy, just go into scenario editor, double click the driver icon above your engine, click the service class tab and select special. I find the service class special sets the speed limits an average of 10mph higher for mainline tracks.
Hope it helps, if not, sorry.
-Cometcatt