Trains just have so much higher capacity than roads. A single lane of highway traffic can handle about 2,000 cars per hour, and that of a surface street probably more like 1,000 cars per hour, while a track of railway can handle 24-30 trains per hour. Trains themselves can carry 1000-1500 people (albeit not comfortably on the high-end), which means you're looking at 25k-45k people/hour/lane on a train. You can't compete with that by road unless you have tons of buses converging on a dedicated busway for a trunk commute (something like this happens in the Lincoln Tunnel).