As you likely know, a Rail to Trail is a former railway line, converted to shared use for cyclists, runners, walkers, etc. In general terms, the first part of the conversion process involves removal of all of the trackage, ballast, etc. Then, a hard base material of limerock or similar is laid down and rolled smooth. The final step sees the bitumen / asphalt surface laid. Note: I’m not a bitumen expert so there may be a step missing in my generalized description.
In this video, JOM of the Gravel Cyclist crew rides a former railway line after all of the trackage has been removed. That includes the rails, ties / sleepers, dog spikes, etc. However, the ballast still remains, which makes any bicycle ride on this surface a wee bit challenging.