That's not just a taxi problem - also idling buses and diesel trains at Stations too.
First generation diesel trains - with locomotives on the ends of carriages - tended to have their exhaust outlets away from the station centres / unpowered passenger carriages. Many are the pictures of Brush 47s, Peaks, 31s, Comptons and Westerns on the end of train outside the train shed at Temple Meads with a string of carriages stretching back into the depths of the passenger shed. The
HSTs▸ with power cars on the end of 8 car rakes continued this trend.
With the nearly-universal use of multiple units these days, with engines and exhaust outlets integrated with the passenger carriages, engines running during layovers are much closer to the passengers, and much more likely in an enclosed space. The power unit no longer just passes the passenger shed / core passenger area briefly - it stops there, chugging outs its gases and particles. Place I've notice it is the waiting room at Westbury where there are some significant layovers.
For Bristol Temple Meads, it's somewhat of a relief to have the terminating London
IETs▸ at the platforms with no overall roof, and of course it will be still better when the electification to and through the station is completed. Bimode probably makes sense for next generation Cross Country too as even today, Bristol - Manchester services could run on electricity from all the way north from Bromsgrove, and Plymouth - Edinburgh services all the way north from York.
Well off topic - rename or split, perhaps?