Last night I arrived at Hammersmith at about 22:00. Trains were present on all 3 platforms, the sign near the ticket barrier was clearly indicating the next departure to be from Platform 2. Stupidly, I believed it and boarded the train - as did most other people. I checked after walking to the front that the indicator still showed platform 2, sat down and sure enough the train on platform 1 duly departed! Is this a common occurrence or does the sign always say platform 2 regardless? . Is there a better way of finding out which train is next?
I'm not a regular user of Hammersmith, so I have no idea how often something like this happens. The next departure boards at Hammersmith are controlled from a switch in the signal cabin, which is obviously set by the signaller. It is possible that the signaller forgot to change the switch.
There are a few other things that could have lead to the train on platform 1 departing first. The driver of the train on platform 2 may have needed to take a quick Physical Needs Relief (PNR in underground speak), in which case the signaller may have sent the train in platform 1 out first, even though he had not originally intended to, although this is an unlikely scenario. It is potentially possible that a slight problem with the train on platform 2 meant that it took longer for the driver to 'open up' after changing ends, again leading to the train on platform 1 being sent out first.
The only other possibility that I can think of is that the train on platform 1 was heading into the depot, but this would obviously be ruled out if it had any passengers on it!
I haven't been on the District platforms at Upminster recently, but when the "next train" was signaled to depart the next train signs switch to the next train. I always checked it when I was on the island platform when the now official "next train" was sitting at a red light, but often the other train was still sitting there with a green signal. Of course if platform 5 was involved, running up and down the stairs you would be likely to miss it. My legs were quicker in those days, now I often use the lift.
Occasionally they realise their mistake and make an announcement that the next train will depart from a platform other than the one indicated at which point a vast throng will de-train, rush from platform to plat form (always good for a laugh it it's 1->2/3 or 2/3->1), and en-train on the correct units.
I think the signalmen at Hammersmith must spend quite a bit of time snoozing or reading the paper because, apart from the 'next train' shenanigans, (which, I must admit seems to happen less now than it once did), they forever seem to hold hold trains at a signal just before Hammersmith and, when we are eventually allowed into the station it's empty! This seems to happen more than 50% of the time in the mornings. Unless there is something in their schedule that says that the next train into the station must use the platform just vacated by the last leaver - which is a quite different piece of daftness.