Start/stop/forward/reverse

[stockers]

You need a double pole double throw switch as a reverser. As its title says - it switches both wires both ways, i.e. reverses them.
Sure Maplins and similar will have more than one variety.

 

[whatlep]

At the moment the control switches on my 2 battery operated locos and enable them to be at stop or in forward movement. What kind of switches do I need to introduce reverse and where can I get them?

DPDT switches with centre-off are what you need. Any seller of electrical switches will have them, but eBay is the obvious source if you aren't near a retailer. Anything like this will do fine:
http://www.ebay.co.uk/itm/10x-Minia...al_Components_Supplies_ET&hash=item20b5fb383a

 

[Madman]

May I suggest Gscale graphics simplest critter control.
http://1stclass.mylargescale.com/DelTapparo/gscalegraphics_1_018.htm

 

[tramcar trev]

You could also try the wireless speed control as discussed here for not much money but I saw how someone set that up for battery....Its cheap and does the job...