As IndustryInsider says, these yellow lights are used by staff. They're known as Cantrail Lights, and unofficially as BILs/DILs.
An illuminated Cantrail light could mean a loss of door interlock, a passcom activation, a fire alarm or a Track Circuit Actuator fault (there are other indications in the driving cab which help identify which is which).
I accept that their purpose is other than to inform passengers that the doors can be opened, but that is how passengers perceive them, especially if they are used to travelling on the tube. I've seen trains being delayed as passengers try in vain to open the doors on a locked out set, in fact I've done it myself before now.
The clue of course is where the Driver stops (if there are different stop boards for different train lengths). If a 5 car comes in and stops by the 2 Car Board, then that's the clue that the back 3 are locked out of use.
This was platform 1 at Twyford, where the only stop board is a "
HST▸ Stop". The train went all the way to the end of the platform.