One way to travel between the two cities is by taking the NJ Transit train from Trenton to Newark Penn Station and then transferring to the Amtrak Keystone train to York. The total trip time for this option is approximately 2 hours and 30 minutes and the cost is around $30.
Alternatively, you could take the SEPTA train from Trenton to Philadelphia and then transfer to the Amtrak Keystone train to York. This option would take approximately 3 hours and cost around $38.
The third option is to take a bus. There are several bus companies that offer service between Trenton and York. The total trip time for this option is approximately 3 hours and the cost is around $35.