The first time I road the C&TS ( in '71) there were strings of cars on almost every siding along the route. To me those gave extra interest as we went by, much like what you are talking about.
IN the scenario you discribe, the passenger train is moving, and the freight is stopped, so why would you need an engine, anyway? OTOH, leaving a string of cars out on a siding probably has it's own risks.
When I road in '07, there was a maintenance train out on the line about every day. Granted, it was usually powered by a diesel, but it showed that there was real activity going on along the railroad.
SInce many people ride the railroad to Osier and back, some will see these "other trains" before lunch and again after, and others won't see it at all. The ones who opt to ride from end to end will get to see it once.
Just my thoughts.
Phil