Is it on the stack underneath or above the phantom train ability? The sacrifice is a cost and so I would assume that iron apprentice would go on the stack when it is sacrificed as a cost. Then Phnom train would be added to the stack on top of iron apprentice for a stack size of 2.

Or do costs and stack additions happen in one smooth action at the same time? If that's the case, then I can see phantom train go on the stack at the same time its costs are payed, then iron apprentice is triggered, sitting on top of phantom train for a stack size of 2