Many different and somtimes contradictory rules have applied to operating different types of equipment on the same tracks. If a railway museum is what is defined as "insular," no FRA rules apply. To simplify the rules an insular line has no connection with the national transportation system, does not cross any public roads or navigabile waterway.

Federal rules don't always mean what they appear to say. Interpretation may have been changed by litigation. FCC rules are notorius for this. In some cases waivers may have been granted.

Most museum operators are very aware of the potential problems caused by operating "lowfloor" (streetcars) with "highfloor" (railroad) cars. Most museum rulebooks I have seen contain extensive rules preventing problems from mixing these two types of cars. Safety stops when approaching a different type of car are common. Minimum spacing between dissimilar types of cars also are common except inside buildings. Speed limits inside buildings are extremely low to reduce damage if the cars touch.

Some interurban companies raised passenger car floor heights in the 1930's to match freight car floors. You can see preserved cars with spacers added between trucks and car floors to raise floor heights. In some cases cars with a two step entrance have been changed to a three step entrance to make easier boarding the higher floors.

On the subject of track gauge, some franchises specified a non-railroad track gauge to prevent freight trains on city streets. However some early street railways used the same spacing used by local wagon builders. Many city streets were unpaved. In theory local wagons could run on the tracks when not in use by the local horsecar or trolley line. This never worked.

Pensylvania broad gauge 5 feet 2 1/2 inches and Baltimore 5 feet 4 1/2 inches supposedly were local wagon wheel spacing. New Orleans streetcars used Pennsylvania broad gauge. Some competing lines used railroad standard gauge.