Trains at standstill after east coast main line power lines damaged
Disruption is expected to continue until 11pm.
Network Rail said "significant repairs" would be required after wires came down over one of the two tracks near Wallyford.
Trains affected include Virgin Trains East Coast (Vtec) and CrossCountry cross-Border services and ScotRail services between Dunbar, North Berwick and Edinburgh.
Vtec advised Scotland-London passengers to use the west coast main instead.
A ScotRail spokesman said: "Our staff are on route to the trapped train between Prestonpans and Longniddry with water and we are putting a plan in place to have you on the move."
Coaches have been laid on to replace some other ScotRail services.