I frequently catch the last train (0050 from London Liverpool Street)on the way home from work. It is not unusual for this to be "switched" from the "down" line to the "up" line at points between Shenfield and Southend Victoria (the route reduces from 4 track to 2 track operation once the mainline branches off at Shenfield). Presumably there is some reason (maintenance I guess) which currently necessitates single-line working during the wee small hours. Can't say exactly how long this goes on into the early morning as if you miss the 0050 at Liv. Street you have to wait for the 0528 or shell out for a taxi home. Certainly - when i was a kid in the 70s - trains did run all night on this line - albeit at very extended intervals.