The middle lever is to be used with a brake rail to stop and restart the train; see the description here:
Edit: After writing the post I saw you had found out already.
Your tender is a Basset Lowke (not by Bing) one; I assume it is from a Basset Lowke compound 4-4-0 (which might be a clockwork or electric one). Here is a picture and video with a correct tender:
Fred- many thanks for the info on the tender and the levers. The linked site is great! Much appreciated.