How long is the tunnel? Is the tunnel on a slope? What is the venting like? Some of the tunnels were so bad that they'd never clear from smoke and had to be electrified. Others weren't too bad. Some tunnels you couldn't breathe for at least a half hour after the last train had been through.
Modern tunnels stink of diesel smoke. I'm assuming that back then they smelled of smoke, whether coal, wood, or oil.
Best of luck,
Jim Clark-Dawe