That article is wrong, it was the first (empty) train of the day (hence the lack of an accident report), I used to work with the driver concerned, he has quite a clear memory of it.
The day before this incident the Shepperton branch was cut off due to engineering work so they decided to use it for low adhesion trials with the 508s, To lower the adhesion available they had covered the rails in all sorts of slippery liquid but when they finished they didn't bother cleaning the rails, the result was the first train approached Shepperton running on good railhead up to that point but on encountering the contamination the train just carried on through the buffers and over the wall.
No rheo on them.