Designing curves for 90 is a good move. It'll need more r/w but the land won't be cheaper later. As for the rest, unless you're willing to build a completely separate railroad (see: CAHSR? the added tracks required in the Chicago-St Louis corridor?), speeds over 79 are simply incompatible with a need to run freight trains under present US conditions. In special cases, where there are large windows with no passenger trains, it might be possible to get up to a 90 mph speed limit like on part of the Surfliner and a few spots on the old Santa Fe. BNSF has the advantage that they already run some freights (priority intermodals) at up to 70, so the difference in speed with passenger trains at 90 is less than it would be on most other US railroads.