Well, actually, the size of the train has nothing to so with the speed of the train. Gravity acts on all objects the same way, regardless of mass. The only difference changing the mass of the train would produce is less inertia. Let me explain:
Having less mass will cause the train to have less momentum, which is actually another word for moving inertia. While this decreased momentum will have no effect on the overall speed of the ride, it WILL make a difference in the total effect of external forces (wind resistance, friction, etc).
You could reduce the train to one car, and still have the same forces at transition points. You'd still get headbanging if it existed before modifying the length of the train because you're going to be moving at the same speed -- but only if you were sitting in the middle of the train beofre its length was modified. Once again, let me explain:
When a coaster is designed, the focal point for all physics calculations is determined for the middle car, or the point halfway through the train. The ends of the cars actually can travel through different parts of a coaster at different relative speeds than the middle. Ever ride a coaster in the front, then ride it in the back? Two different rides because there are slightly different forces being applied to your body. The lift hill is the easiest example -- the fromt car is usually hanging there until the rear is released from the lift. Then it is seemingly "pushed" down the hill by the rest of the train. The rear, however, as soon as it is released gets "yanked" over the top, in some cases rusulting in a little airtime for the rider. This also happens on other hills. Depending on the hill, people in the front will get airtime while getting to the top of the hill, while people in the rear will get it when going down the other side of the hill.
So, to get back to the point at hand... Reducing the length of the train, in some cases, might result in an 洀瀀爀漀瘀攀搀 爀椀搀攀Ⰰ 戀甀琀 䤀 琀栀椀渀欀 琀栀愀琀 琀栀椀猀 爀攀愀氀氀礀 眀椀氀氀 戀攀 搀攀瀀攀渀搀攀渀琀 甀瀀漀渀 琀栀攀 爀椀搀攀爀⸀ 䤀昀 礀漀甀✀瘀攀 渀攀瘀攀爀 爀椀搀搀攀渀 琀栀攀 挀攀渀琀攀爀 猀攀愀琀 漀渀 愀 挀漀愀猀琀攀爀Ⰰ 椀琀 眀漀甀氀搀 猀攀攀洀 搀椀昀昀攀爀攀渀琀⸀ 䤀昀 礀漀甀 愀氀眀愀礀猀 爀椀搀攀 椀渀 琀栀攀 洀椀搀搀氀攀Ⰰ 椀琀 洀愀礀 渀漀琀 戀攀 渀漀琀椀挀攀愀戀氀攀 琀漀 礀漀甀Ⰰ 猀椀渀挀攀 琀栀攀 洀椀搀搀氀攀 漀昀 琀栀攀 琀爀愀椀渀 椀猀 甀猀甀愀氀氀礀 琀栀攀 洀漀猀琀 ∀挀漀渀猀椀猀琀攀渀琀∀ ⴀⴀ 洀攀愀渀椀渀最 琀栀愀琀 椀琀✀猀 眀栀攀爀攀 琀栀攀 昀漀爀挀攀猀 愀爀攀 昀攀氀琀 氀攀愀猀琀 ⴀⴀ 爀椀搀攀 漀渀 琀栀攀 挀漀愀猀琀攀爀⸀ 㰀戀爀㸀 㰀戀爀㸀䠀漀瀀攀 琀栀愀琀 栀攀氀瀀猀 礀漀甀 愀 氀椀琀琀氀攀⸀ 㰀戀爀㸀 㰀戀爀㸀匀漀爀爀礀 昀漀爀 琀栀攀 氀漀渀最 瀀漀猀琀⸀⸀⸀⸀⸀⸀
