Philadelphia’s 30th Street Train Station to San Jose, Costa Rica for only $250 r/t with Amtrak and United Airlines.

The itinerary begins at Philadelphia’s 30th Street Train Station where you will board an Amtrak train to Newark, before taking the AirTrain to Newark airport (all included in your ticket).
Here, you will board a direct flight to San Jose, Costa Rica with United Airlines.

You MUST check in and board the Philadelphia-Newark train at the beginning of the journey.
You can, however, skip Newark-Philadelphia train at the end of the itinerary.

There are many dates available from October to March. The price are going up gradually. Just got my booking confirmation and my $250 ticket includes the $29 exit tax as well.

Oops, almost forgot. Use Google Flight to search your dates. Philadelphia (all airports) to SJO

https://www.google.com/flights/#search; ... 2015-10-28

interesting... EWR to SJO direct for that day is 415$... i wonder if i could just book and fly from instead...
is there something in writing that says one must check in at Philly?

There doesn't need to be anything in writing. It's a connection. Once you fail to check in at Philly, the ticket is canceled. Same as a flight with a layover, but this first leg is a train ride instead.

It's the same as if you had a ticket from EWR to IAH to SJO. You can't just get on the plane in Houston if you didn't board in Newark. Once you miss the first leg, the ticket is canceled.

On the way back, you can just get off in Newark and not take the train back to Philly. Nothing they can do since you have used up the ticket already. But next time you book with that airline, you may have some essplaining to do.

Even if you have to take a train to NWK from PHL, it only takes an hour and costs $41. That is still a good deal. This may be a good alternative from the DC area also.