Can i travel from Russel Square to Eltham with oyster card?How much will it cost?Wont i need a railway ticket?

Answer 1

That depends what you have on your Oyster.
If you have a weekly travel card, then you can use it for the train.
If you only have Pay As You Go then you will have to pay for the train, or take the bus. You could get the Jubillee Line to North Greenwich and then a bus or two depending where you need to get to.

Answer 2

You can travel from Russell Square to Charing Cross using an Oyster card, and then I think you would need to buy a train ticket to go from Charing Cross to Eltham by taking the Southeastern towards Dartford Rail Station.