These are the newer vacuum toilets found on Midland Mainline Meridian Trains, the older vacuum toilets have a more goldy effect. The even older toilets are ceramic and flush like a normal toilet but drop through onto the track once it goes through the u-bend.
These ones flush by spraying water round the bowl to push all waste by the hole (from the 5 nozzels around the top of the bowl) and creating a vacuum. After enough pressure has been created it will suddenly suck the contents out with a loud bang. If anything gets stuck in the hole it will start to create another vacuum more powerful and suck it all out again if that doesn't succeed then it goes again and so on.
The flush button has lights to indicate when the toilet is ready (when you press it the lights go out), if it gets blocked and hasn't cleared it fully out the hole then the lights stay out until the pipe is clear. Pressing it while lights are out won't do anything. The button is usally located to the left of the toilet.