Stansted has a train service to and from the Airport, terminal to terminal meaning if you get the train from Stansted or London you will get a seat on a train with plenty of room for your bags on new state of the art trains designed for travel to and from the Airport with wifi, You can evn get trains to Stratford, Cambridge and Birmingham.
There are also coaches operated by many companies who have many services from London and other UK cities to the large coach station at the Airport.
What will you get at Southend ? A packed commuter train where you will struggle to get a seat at peak times or a train full of hoodies going to and from Southend to play on the slots and get Pi**ed.
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