If you're taking the time to build a tunnel, might as well put a subway in it. Come to NYC (or London or Paris or ....) and Chicago and see real subways are a much better use of building tunnels. If NYC, Boston, DC, SF, Philly, all have subways, why not LA?

It is a much greener solution than using electric autos which after all, must be charged with electric power which is often carbon-based.

Subways are also far faster than using automobiles during much of the day but especially during Rush hour. Much of the NYC subway system is powered by Hydro Quebec. As stated earlier, much of even electric autos are charged by electric power generated with carbon-based fuels (e.g., natural gas).

Building a subway in LA would have a much lower carbon footprint than building a tunnel for cars.

Trump comes from NYC, and while he probably doesn't take the subway, he might likely understand the importance of them.