Testing workshop is a location in S.T.A.L.K.E.R.: Call of Pripyat.
True to its name it was used to test the prototype of the Gauss Rifle. The prototype itself is still in the facility. There is also a train car with a solid thick slab of steel which served as a target to test the rifle's power and monitoring/control stations positioned above the testing area.
Because of the train car one might assume that either the facility is much larger but most of it inaccessible, or that there is an underground rail network running all the way to Jupiter Factory. The Iron Forest itself was most likely an electrical substation used to power the prototype.
The player will visit the Iron Forest for the first time investigating Stingray 2; a Blowout or curiosity might direct him to the facility's door, but accessing the facility itself will happen much later in the game as part of Col. Kovalsky's quest to investigate the Gauss Rifle. To open the blast door, the player must obtain the access card from Cardan. The facility itself is very small, in fact it's not even a separate level - it exists as a part of Zaton geometry. The top level is occupied by several Zombified stalkers(including the Exoskeleton variety), and the prototype itself is guarded by a Pseudogiant. Thankfully, some grenades and rare ammo can be found scattered in the control rooms, as well as on zombies themselves.
- (1.6.02) Due to a physics bug it might happen that the door to the facility will be open before the player even visits Pripyat. You're welcome to explore the facility itself, but do not pick up the Documents concerning item 62 before you receive the keycard from Cardan or it will screw up the game's scripts.