I hesitated when this thread first appeared, but will offer this quirky idea for what it is worth.
I often run two trains on one loop without blocks or sidings - but there are a couple of "strings attached" to how I do it. Allan Miller and others commented in a previous thread about this method that I'm not really "running" my trains since in doing this "trick" I have very limited independent control of either train. That is true. On the other hand I have two trains sharing the same track, often for hours at a time, without colliding: depends what you want to do.
The basic idea is illustrated in the diagram below which shows the voltage versus scale-speed "curves" for two hypothetical locomotives. Locomotives A and B have been chosen so they have very different voltage-versus-speed curves that cross one another. If put on opposite sides of a loop, and run at twelve volts - the voltage where the red and green lines cross, they will run together and neither will ever catch the other. Drop voltage slightly - say to 11 volts, and loco B will begin to slowly catch up with loco A. Raise it to 13 volts and loco A will begin to catch up with loco B. You can set up your locos on opposite sides of the loop, set them running and adjust throttle up and down around the 12 volt point until they are spaced well, then leave it at that precise point and just let them run at that one magic speed.
This works for me largely because I have a long loop (141 feet) on which I run two relatively much shorter trains (loco+tender+6 cars+caboose) each maybe ten feet long, for about 60 feet between each train and the one in front. Usually, I can only get the speed dialed in about right, so one gradually catches the other, but I keep watch and adjust ever 30-45 minutes or so, they never touch. This really does work well after a little practice and I do it often but only on my longest loop. It means that I run four trains routinely rather than just three, which makes my train room all that more fun.
If you want to do this "trick," usually locos from different companies work best together. My favorite pairing is an RK PS1 Y6B with either a Lionel 0-4-0 or Legacy Southern Crescent. The Y6B has a wildly different voltage-speed curve compared to the other two, so either piaring works well.
To some people, this is not "running" trains - and I can see their point. I have only one option for speed and can't control them very independently. But I tend to run my trains my just turning them loose to cruise for hours "on their own" so it works for me: as I said, for what it's worth