Northwest Seneca Station

You find this place after killing 2 super mutants and opening a very hard locked door (you can find a key somewhere on the floor in the place).

Here you will find some mole rats guarding the entrance. Once past them you will meet 2 ghouls. Talk to Murphy to get a note that you can bring him Sugar Bombs for 15 caps. He wants it to make SuperJet double strength Jet. You might be able to persuade him to buy them for 30 if you have high speech or other stats.

If you take him some Sugar Bombs he will sell you some SuperJet.

In the room around from Murphy you will find a manhole cover that goes to Meresti Service Tunnel.