One of the reasons for this is that both Mistley and Wrabness stations can only accommodate 4 carriage trains and the regulations stipulate that longer trains are not allowed to stop there. This means that a valuable train path to London is taken up by a 4 carriage train which often gets overcrowded as it gets nearer to London. Whilst the running of a shuttle is very inconvenient for Harwich branch users it does mean that a greater number of passengers further down the line do not lose a service and consequently this is the thinking behind such a proposal.
Unfortunately, physical constraints such as bridges etc. prevent the platforms at Mistley and Wrabness being lengthened. The MRUA will continue to campaign for longer trains to be run on the Harwich branch where the introduction of a system whereby only the doors on, say, the front 4 carriages are allowed to open at Wrabness and Mistley could be introduced as has been done in other parts of the country.