The São Paulo Railway Company runs freight trains that switch between adhesion and rack railway. I watched a video of the changeover and the train doesn’t even need to stop even though it has to travel slowly for that stretch. One of the companies involved says that, “One locomotive can pull up to 425 tonnes, and two in series can manage twice that amount. They reach an uphill speed of up to 30 km/h.” It would appear then that changeovers and top speeds wouldn’t be much of a problem. There are a lot of other hybrid systems but they are all subway or light rail which are much lighter than a commuter rail train.I think it's a good question. It's the only way you can find out if anyone knows whether or not your suggestion makes sense.