There are several solutions that work. I like simplicity without adding more items to look at. I can run 4 trains at at the same time on my layout. There are three mainlines, mainline 1, at 190', accommodates two trains. As Vern suggest, how the engines are assigned TMCC ID's is a good start. Unfortunately in S gauge the numbering system cannot always be just the last two digits because of the cab numbering overlaps between American Models and Lionel engines. I use the first two digits of the AM engines and the last two of the Lionel engines. There are still about 10 conflicts which need a one off ID assigned.
To run four trains I just use two Cab2's since each Cab2 toggles between two engines. The two trains on line one are on one of the Cab2's, the trains on Lines 3 and 4 are on the other. Works for me. If I need to quick look up an engine ID the entire roster is listed on the iPad screen in the LCS "Roster" view.
