One other aspect that affects the speed over the Mile End to Stratford Central line link is the this type of Automatic Train Operation (Westinghouse UK) in use from the 1990s line upgrade. This system only supports a limited number of speed codes. These are the maximum safe speed that can be assigned to the train for any given section or movement authority. Because of the need for quite a few low speed codes to reflect speeds through curves, switches and crossings along the route, there are no speed codes between 65 km/h the maximum tunnel section speed and (originally) 100 km/h the maximum line speed. The tunnels east of Liverpool Street are much newer than those west of that point and are generally much straighter and have fewer steep gradients, and could be made suitable for higher speeds than 65 km/h.
In the early stages of evaluating options for the Central Line Improvment Programme (CLIP), increasing the speed across this and similar sections was considered in order to reduce run times across a very busy part of the line, but the highest speed code (now 85 km/h) would have resulted in too many gauge infingements (as the vehicle moves about more at that speed), potentially causing it to come into contact with something (for example cable runs) without suitable works to fix. The likely cost of either suitable gauge enhancements and / or a signalling modification to provide an additional intermediate speed code (75 km/h for example) were both understood to be too high to justify inclusion in the project given the budget available, despite the potential benefit.
It is however a constraint that any future upgrade including significant modification to the signalling and / or new trains would want to look at again, as there is a clear opportunity for future imporvement in the speed here and across other tunnel sections between Liverpool Street and Newbury Park...