Noted on an East Midlands Trains Class 153 Sandbox Lid at Lincoln earlier this year.
Why would anyone think to use anything other than 'dry sand' or do E.M.T. consider their Fitters to be somewhat challenged?
Speaking as a former Railway Fitter it makes me wonder.
UPDATE: This from Dogboxdriver...
The signs were fitted after a number of EMT's 15x units were failed by drivers during preparation over the last year owing to non-working sanders.
The reason for the non-working sanders?
You got it: they'd been filled with wet sand...