I'm assuming the answer to this question is no. Is a exit ramp off of / or to a city street from / to an interstate allowed to cross a railroad track.
Yes it is allowed, though for obvious reasons, it's not preferred. As an example, the EIS plan for the I-494/CSAH 28 interchange in Bloomington, MN calls for the westbound on-ramp to cross the adjacent rail line at-grade. This became the plan because alternatives that avoided the at-grade crossing resulted in unwanted consequences (excessive right-of-way needs or tight ramp curvature).
Meanwhile, numerous folded-diamond interchanges were built nationwide due to the preference to NOT have the ramps cross a rail line. Many (most?) remain even if the rail line adjacent to the side road being interchanged is no longer in service.
thanks
Not (yet) an interstate, but the ramps on US 10/WI 441 to and from Racine St in Menasha, WI all cross a minor railroad spur at grade. The rail line serves a couple of industrial customers (switched daily) and the level crossings will all be retained when the freeway is upgraded to six lanes in a few years.
http://goo.gl/maps/0qQzo
Mike