(19 Nov 2020, 10:32 am)RMF1254 wrote Northern’s National 2s were all delivered with roof pods except 4691 which I believe was a diverted order (East Yorkshire comes to mind but I’m not sure). United’s new National 2s were delivered without the pods but I think some of their later second hand ones had them. The podless National 2s were a more basic model with a different heating system I think.

I've had a quick look on wiki and under the Mk1, they mention the 10351B/1R model (without the pod) was 10.3m long, had different lighting and had underseat heating, as opposed to the heating coming through at roof level and the option of an 11m length.

I assume the same applied to the Mk2's, with 4691 and United’s examples having the underseat heating.
It's not something I ever consciously noticed as a passenger at all. 

When the ex-Northern examples lost their pods later in life, I am guessing the heating system was changed at the same time.