Stations. Norwich Diss Stow market Ipswich Manningtree Colchester Marks Tey Kelvedon Witham Hatfield Peverel Chelmsford Ingatestone Shenfield Romford Stratford London Liverpool Street Trains. Class 315 not sure if this is the correct livery Class 321 Class 317 Class 357 Class 379
Based on Brighton Main Line with 37 miles length, a section up to Witham would be reasonable (38 miles something).
I used the data from Wikipedia in Germany, where it states something about 60 kilometres. But you are correct. In that case Colchester would be an appropriate section end (assuming same length as BML).