billybigrig:
Is that not just like the sand box on early steam trains that dumped sand in front of the dive wheels. I’m sure someone with an anorak will soon confirm this
Yes, and I believe they are still used on current trains, especially during the “leaves on the line” periods. I think I’ve just been “outed” as an anorak… 
selby newcomer:
Thank you, I have seen the switch your talking about and have wondered what it’s for, I think on my unit there are 2 switches with axles on, would 1 be to turn it off and the other be to turn it off??
A difflock switch is usually a picture of an axle with a cross through the centre (where the diff is!); sometimes there’s what looks like a “wheelplan” again with a cross through the centre of the axle(s) that are locked by that switch.
See here:
The left button controls the lift axle; the right one controls the weight transfer function, and the centre knob controls traction control and difflock. The knob can be turned to three positions - “off”, “centre diff locked” and “front, centre and rear diffs locked”. This one appears to be from a 6x4 unit (i.e. two rear axles, both of which are driven), and that’s why it’s got two “on” positions.
Most units will only have one driven axle, so the choices won’t be quite so complicated.