The T-55AM is an unlockable tank in Red Crucible: Firestorm. It is earned through completing the "Steel Fist T-55AM" mission, which requires the completion of the "Steel Fist Super Sherman" and "Steel Fist T-34" missions.
It is essentially the same thing as the Super Sherman, albeit with somewhat better armor protection on the turret. However this extra level of protection on the turret is useless, as the hull is most commonly the first targeted area when fighting, unless you're camping on a hill in a hull-down position. The gun is, however, slightly more accurate than the one on the Super Sherman, making it more suitable for sniping tanks at a long distance. Other than that, the T-55AM is somewhat useless, but it's still worth keeping until tanks like the T-64A are unlocked.