Suzie

Liverpool Street to Shenfield has had three different voltages, 1500V DC, 6.25KV AC and 25KV AC

Damage to silicon devices can be cumulative. When a silicon junction is subjected to a higher voltage than it can withstand the insulation breaks down and suffers a small amount of damage. Eventually when subjected to enough abuse the insulation becomes sufficiently compromised that the device fails - usually as a short circuit. Just because a decoder survives once does not mean that it has been properly protected. Even Zener diodes can be a little slow to react to a sudden application of high voltage and fail.

It looks like it should be OK in practice since it is current limited to 2mA and most decoders will draw more than that even while idle (typically 10mA or more) due to the regulator maintenance current. You could put a 4K7 resistor between the red and black wires of your decoders to make sure that they will not go above their safe voltage while 2mA is applied if you have any that do not draw 2mA. The theory is sound as long as it does what it says it is supposed to do! The mobile version works and is a lot less risky but does not fit the loading gauge very well (too tall and the steps foul the bogies).