Used to do it almost daily back when I was a signal maintainer in Los Angeles. Had a Hy-Rail truck I took home from work. Was quite fun.
The steering wheel will have some kind of lock so you can't turn the wheel while on the rail (Pro tip: don't forget to unlock it when you try to exit the track

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The rail wheels are just guides, your road wheels will still propel the vehicle.
Would assume fuel economy would be a little better, but I only did ~15 miles at most at a time and never paid attention. I do know it's a hell of a lot faster to get from Los Angeles to Long Beach during rush hour using the rail tho

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Don't have one anymore, but I did receive this today while I was trying to perform commissioning tests for Detroit's new streetcar.