You can use your pass on basically every (mainline) train in the UK, of about every company. Only not on Heathrow express, but I’m not sure. It’s easy to skip anyway.

It’s a real pleasure, interrailing through the UK.

In London the Elizabeth line and the London Overground are included.

Underground, Busses, Tram, DLR are not included in the pass.

Here’s the list of the mainline companies included:

https://www.raildeliverygroup.com/uk-rail-industry/passenger-freight-track.html#tocs

Hello Travi

If you plan to explore London during your Eurail trip, I would recommend getting Oyster card or use contactless debit/card to travel. 

Just wanted to add my recent experience in London (May 2023). Eurail works with the Elizabeth Line and all other trains in London except the Tube (and Heathrow Express, which is basically the Elizabeth Line without stops).

But the implementation isn’t great. I used the Rail Planner app to go from one station to another, and it displayed a QR code on my phone. Supposedly, you should be able to just place your phone onto the turnstile’s code reader, and gain entry. But 1) relatively few turnstiles had code readers, and 2) even when they did, my QR code never worked. It always said, “assistance required.” I had to call over someone in a vest, they looked at my pass, and then manually unlocked the handicapped turnstile to let me through.

So by all rights, the Eurail pass works in London, but not the way it should. You need a worker to manually let you through at every turnstile, both entering and exiting. This can be a problem if you’re travelling to small stations that have few workers, or sometimes none. I went to Norwood Junction to visit Selhurst Park, and there was one worker behind glass, whose attention i couldn’t get. I had to jump the turnstile, then sheepishly explain myself to her afterwards. Again, all transport workers should recognize a Eurail pass, but many don’t. You have to be persistent, because you paid the money and your pass is legal.

As others have mentioned, you probably also need an Oystercard to ride the Tube (and buses/trams, if you’re into that). Indeed, many Londoners have now taken to just using their contactless credit cards, or even their phone at the turnstiles. But if you’re careful with your planning, you don’t have to put as much money on the Oystercard, by utilizing your Eurail pass whenever possible.