If you are travelling to Ely from London, there are a couple of ways you can do it. There are trains to Cambridge from either Liverpool Street or Kings Cross, and trains to Ely from Kings Cross.
The trains from Liverpool Street are stopping services, as is the Ely train from Kings Cross.
If you time it right, I find the quickest way is to get the non-stop train from Kings Cross to Cambridge (50 minutes) and there is usually a connecting Ely train on the adjoining platform. I managed the entire journey recently in 70 minutes.
I find the website shown extremely useful. It covers all train services in Great Britain
Ely is not a very big town, but its train station is important. Several lines go through Ely and several train companies use the station. When my direct train from Cambridge to Norwich was cancelled because of a strike, I was very glad when I was told I could go to Ely and change into a train run by a different company there.
From the train station to the town centre it's a short walk. It's something special to apporach Ely and see the cathedral from the train window, how it's getting bigger and bigger.
There's an excellent car park on Barton Road. It's big, its free (up to 23 hours), it has toilets and it's only about 5 minutes walk from the cathedral. Follow the signs for 'long-stay parking' from the A10 Cambridge road.
There are other long-stay car parks too (Broad Street, Ship Lane) and short-stay in St Mary's Street/ Newnham Street, but Barton road is by far the most convenient if you are coming from the south or west.
Wonderful, over-the-top English bungalow gardens next to it (the English flags are because of the World Cup)!