The goal was to help 50k people commute between two cities. One potential solution is to widen the highway by one lane on both sides. One other solution is to setup a dedicated commuter rail. The money was being spent for commuters, you are basing cost to commuter capacity ratios. Even if an HSR train could take 50k people is one go, that still doesn't make it better. In reality, everyone will not show up at the same time. In reality, the 50k people that do commute, not all of them could use the rail due to their occupational requirements of tools/material transpo and/or needed to travel throughout the city during the work day. In reality, operational costs out weigh the benefits.
An HSR is like the Gravina Island Bridge project, aka, Bridge to Nowhere: https://en.wikipedia.org/wiki/Gravina_Island_Bridge
A $400m dollar bridge to replace a ferry for a town of 50 people. Yes, the bridge has a better capacity than the ferry. It still isn't useful because of all the costs associated for so few people.