This. :)

What's interesting is The Cascade Tunnel. Longest RR tunnel in the US.
https://en.wikipedia.org/wiki/Cascade_Tunnel

"Ventilation operations

Because of the length of the tunnel, an unusual system is used to ensure that the air inside remains breathable and reduce problems with excess fumes. For example, as a train enters the west portal of the tunnel, doors close on the east portal and huge fans blow in cool air through a second portal to help the diesel engines. As long as the train is within the tunnel, the fans work with reduced power to avoid pressure problems.

After the train has left the tunnel, the red-and-white-checked door closes and the fans operate for 20 to 30 minutes with maximum power to clear the tunnel of exhaust before the next train passes through. In the opposite direction, the door opens when the train is within 0.6 miles (1 km). The fans are powered by two 800-horsepower electric motors, clearing the air through the seven miles (11 km) of tunnel within 20 minutes. Present-day train crews carry portable respirators for use in the event of a fan failure or a train stalling inside the tunnel. In addition, there are emergency/safety stations spaced 1,500–2,500 feet (460–760 m) apart, depending on the location within the tunnel, that provide additional air tanks and equipment to be used in the event of a ventilation/other failure.

The tunnel door is protected by an absolute signal near the east portal; at Scenic on the west side, another signal with dual flashing lunar aspects indicates to eastbound trains that the tunnel fans are operating.