Wow… I didn’t know the railroads would allow extra riders in the locos. I heard US railroads would sometime ‘deadhead’ crews in the rear locos if they needed to get back to home base from some remote locations.
Some long tunnels have extreme ventilation systems like the Moffatt tunnel above Denver on the continental divide. There are roll-down barndoors and huge ventilator fans at each end of the tunnel that perform mandatory ventilation after each train passes. The next following train must wait in the siding until the tunnel clears before being allowed through.
Imagine the old days of steam locomotives going through these long tunnels. Back when a caboose was mandatory, the poor guy at the end of the train had to endure a bit of tunnel gas also, although the long train ahead helped clear the smoke a bit.