Mark 4 Coach - Underfloor Tank

[Norther Monkey]

Currently sat adjacent to a LNER Mk4 set. I noticed on coach B a tank, below floor level, roughly centre, with a contents gauge. It didn't look like water or waste for the toilet.
Satisfy my curiosity, what's it for?

 

[Darandio]

Whereabouts on this picture? https://flic.kr/p/Gx5AJT

 

[Norther Monkey]

Just to the right of the left hand bogie in the picture. The tank seems to stop at the vertical space to the next piece of equipment. There's what appears to be a filling point in a recess on the left, and a level gauge towards the right hand end.

 

[aleggatta]

Looks to me like a standard CET tank, with the gauge probably unrelated for main res or brake pressure, but not having seen the gauge up close and personal I couldn't say for certain.

 

[Norther Monkey]

The gauge was oscillating following the train stopping, which made me think liquid as opposed to air for brakes.
Regarding CET, the 'access' point seemed to be towards the top of the tank. I would have expected CET to have a drain at the bottom.

None of which is conclusive, but it's giving me something to ponder as I travel!

 

[aleggatta]

Some CET tanks have internal pipework that creates a collection point at the bottom of the tank (which is very frustrating when the pipework gets blocked). Many times I would have appreciated a bottom drain point on the tank for maintenance purposes! Not sure I could give any conclusive answer with regards to the oscillating gauge, but it might be Air suspension (which would be constantly adjusting to maintain ride height) if passengers were alighting/boarding the train.