I have seen this led system right from the time when it was first tested on a few trains by SCR in 2009. First Sabari express used to come with led and most of them working when rumming form HYD to TVC. the return train had many boards with just welcome message.I noticed this as both sabari meet around 12:00 midnight at TPTY Then it was tested on 1747/88 tirumala express at TPTY. A staff from JE CDO dept used to carry a hand held device actually it was quite big like a coin phone device and used to feed the train details by going near every bogie and entering bogie number and corresponding train number. the range of the device was 2 to 3 bogie length. when ever there was power problem in a particular bogie or the power is switched off it automatically resets to display welcome message...
more... and has to be feeded with the train number again and because of this many coaches of 17488 used to come with just the welcome message on third day. Now with so many rake sharing and some time very little turn around time and also turn around from many small station these errors are bound to happen. The facility to change train number is generally available only at station which has a working CDO and maintenance of some rake. at other stations these facility are not available and hence we often see the welcome sign instead of train number and station names.