As a guard I find when its really hot is the worst weather to work in , no matter how many windows you open or how hard you will the aircon to do its thing you just cant cool the trains down . You end up with instances of passengers fainting because of the heat which is never a nice situation to be in . And generally heat sends a lot of passengers a bit loopy . Not to mention there being nothing worse IMO than getting a soaking wet sweaty £5 note that has just been in someones back pocket all day . Also the sun is a real hindrance to carrying out station duties especially when its low in the sky .
I dont mind when its cold or wet because you can always wrap up to keep warm , the only cold weather that is a real problem is the snow or ice which makes platforms a nightmare . Because of the shifts you can be the first person to be driving on snowy or icy roads because of the times you drive to/from work so that can get a bit hairy in the depths of winter .
I imagine for drivers fog can be bad for exposing any gaps in route knowledge , and when the sun is low in the sky it can be a nightmare for sighting signals , I know it is a hindrance when trying to carry out station duties and sighting starting signals so I cant imagine how bad it is when hurtling along at 75+mph .