Modern cars have a feature whereby when in traffic jams, the air system can be switched from sucking in external air to re-circulating air to prevent pollution entering the vehicle. Do trains have a similar feature and if so, do drivers use it? (I have smelt diesel fumes when in tunnels although on HST’s, I suspect that much of this enters through door windows left open. It is regrettable that Box tunnel between Chippenham & Bath has the electric conductor rail but will not be linked to the electric system for some time. It is also regrettable that apart from London trains, the Severn tunnel will still have an entirely diesel fleet going through it. I just hope that the Turbos will have their windows closed and a working air-con system in operation).