I've often wondered why trains have to come almost to a stop rather than just passing through at line speed as would happen on the 'big railway', something to do with the tripcock?
On Saturday I was on the eastbound platform at Sloane Square and the westbound platform was closed because of building work upstairs. Most trains were approaching at a crawl, far slower than if they were stopping at the station normally, and some were sounding the whistle even though there was nobody on the platform. One particular train came in a bit faster and came to an abrupt halt with just the front two coaches alongside the platform, it sat there for a minute or two and I noticed the driver on the phone. Had the train been 'tripped' and he had to get permission to continue?
The main reason is simply that signal overlaps are generally calculated on the basis that every train starts from a platform from a rest. If a train were to pass through a platform at full speed then the calculated overlap lengths no longer hold true. It's also undesirable as many station starter overlaps are of nominal length only - on the assumption that a misjudgement is more likely than a driver making no attempt to stop at a platform at all. There is a caveat however that this will not apply if there is a signal berth in advance of the platform where a train can be brought to a stand. Certain designated platforms are designed for running through at higher speed, some might be 25 mph for example, others might be full speed.
Another reason for running slowly through platforms is that the starting signal may not have sighting from full speed, as starting signals generally aren't provided with a repeater. A platform repeater, if available, is not sighted for being seen by the train operator, plus unlike a normal repeater signal they will show green even if the associated stop signal still displays a dual aspect. Repeater signals are provided for starting signals at the specific platforms where trains are authorized to pass through at higher speeds, an example of where this may be found is between Ealing Common and South Harrow.
The difference between LU and mainline is that LU's overlaps are designed to be able to allow for a train to stop, whereas on the mainline they are normally provided solely to cater for a misjudgment. The lower speeds on LU make this arrangement feasible, whereas on the mainline it wouldn't be feasible to provide 125 mph overlaps whilst maintaining line capacity.
The ATO lines are different as with continuous ATP there's no need for any restriction, however evidently LU feel that there should still be some kind of restriction - for safety of people on platforms plus other issues like displacement of tunnel dust. On the Central Line there's nothing to physically stop a train going through a platform at full speed, however the rules say otherwise.
Historic practice was that drivers were generally instructed to stop short of a platform if the starting signal was at danger, however over time many starting signals have gained timing sections, so this practice has tended to fall into disuse. It doesn't apply on the TBTC lines due to the design of the system.