M1A1 Abrams

The M1A1 Abrams is a main battle tank produced in the United States. It is a well armed, heavily armored, and highly mobile tank designed for modern armored ground warfare. In Half-Life the Abrams is first encountered in the chapter Surface Tension, providing fire support for a group of entrenched marines. Another one was stationed near the end of "Forget About Freeman!", blocking the entrance to the Lambda Complex. It differed from the first in a way that it fired a machine gun as well.

In Half-Life: Blue Shift, a single Abrams was encountered in the chapter Captive Freight. It was placed on a train wagon, with a nearby truck conveniently stocked with rockets. However, if the Abrams catches the player there, it blows off the truck's back which can only be survived by crouching. Curiously, this Abrams only takes damage from hits on the turret. The Abrams is not encountered for the rest of the game.

Tactics
The Abrams is completely bulletproof, so you will need explosives or experimental weaponry to destroy one. It will keep on firing you with its cannon and machine gun, so use cover and shoot it with your RPG or the Tau Cannon's secondary attack. If you get close enough, you can also drop some Satchel Charges, but be sure to get clear of the blast!