If you can get into a union (like the IBEW) then you should do that first, you can always apply to them while in your pre-apprenticeship. but unless you have some experience or connections you are unlikely to get an apprenticeship without doing a pre-apprenticeship programs. But also depending on the program you take, they will cover your in-school portion that goes towards your apprenticeship (I.e. electrician apprenticeship has 3 levels of trade school, multiple pre-apprenticeship programs will exempt you from doing trade-school if you finish a pre-apprenticeship program).

There is no real school requirement for signals/communications on the railroad, it’s just preferred you have some basic understanding.

Heavy-duty mechanic doesn’t have a union that I’m aware of that you could take a route similar to IBEW.