It's something like that. For my example of Iron Rattler, consider this related situation: Diamondback's trains on Valravn. I've not done any calculations, but I'd imagine that drop creating unsafe forces in the back of the train.
Well, not really. In the case of the bottom of a drop, longer train cannot increase the forces, but quite the contrary actually : On a short (dive-like) train, the whole train reach the lowest point at the same time, so each passenger reach the bottom (and the point with the highest g-force) approximately when the centre of mass of the train is at its lowest, and the speed at its maximum. The g-force is maximal for everybody.
If you take a longer train, when some given row hits the bottom, all the previous rows already started climbing the Immelman, while the following rows are still on the drop. The centre of mass of the train is therefore higher, the speed and g-forces, lower.
As the centre of mass of the train is the lower when the middle of the train reach the bottom, the strongest forces actually apply to the middle rows, not the back or front.
In the case on Valravn, and more generally in any valley, longer trains reduce the forces, especially for the front and back rows.
Longer trains increase the positive g-forces on the back row during a double down (and decrease the forces on the front row), as the centre of mass of the train is lower when the back row reaches the bottom of the first dip. It increases positive forces on the front row during double-ups for the same reason.
On a hill, longer train increase the airtime at the top for both extremities. For the front it increases airtime on the uphill but decrease it on the drop. For the back, it decreases airtime on the uphill and increase it on the drop.