hrfcarl wrote:I do not believe there is a FRA requirement for seperate ROWs for freight and passenger, otherwise NY&A and CP could not operate on LIRR and MNRR ROWs, respectively. That is ignoring Amtrak's use of freight ROWs throughout the US.
Sorry, I wasn't quite clear and mixed my modes. When different modes (freight, passenger, subway, light, et al) share ROW, FRA requirements govern how the passenger cars are built. That's why you won't see freight or commuter on subway or light rail, no matter the operating compatability. That's why you couldn't put freight in a subway tunnel - NYCT would be required to adhere to FRA requirements for their cars, then. I think those requirements concern crash-worthiness.
Of course, you are 100% right on commuter and freight sharing ROW across the country; however, there are more stringent requirements on the commuter cars in those cases. Think Acela. The reason it is so expensive is it does not run on a dedicated ROW; therefore, it must meet more stringent crash-worthiness standards. If you built a commuter or high-speed rail line on a dedicated ROW, you would have less stringent requirements. I think you could conceivably put commuter on subway or light rail; but only if the commuter line itself were a dedicated ROW with less stringent standards. Can you imagine an M-8 with all that weight bearing down on a subway train? The possibility of derailments is why physical separation between ROW is required. That's why, as you noted, you'd need a separate level since freight and transit don't mix.