Half-Life-Power Up

Trying to reach the surface once more, Gordon's progress is thwarted by a clash between HECU soldiers and a Gargantua in the Sector E Materials Transport. He then learns from a dying security guard that if he restores the generator, he can ride the tram to the surface.
Freeman enters the Houndeye-infested subterranean areas and activates the generator, restoring power to the vicinity; after activating more switches, he destroys the Gargantua patrolling the area by turning on the transformers supplying the rail system with power.
Jumping onto a tram, he is able to continue his way by smashing through the concrete barricade blocking the access to the rail system.