This is a great idea ! Too bad, as writeorich says, there are no low fares from Penn Station. The fares are way too high.
For example, to Newark it's about $40 round trip and Yonkers $34.

Another thing I noticed is that the website doesn't allow a passenger to price/pick return options that have 1 hour > between segments. Makes the trip easier so you won't have to wait an one hour or more at the station.

I receive this message
Please allow a minimum of 1 hour between each segment in your trip. Please select another train combination. Thank you.