Your right that more energy will be lost throughout the ride due to the extra rolling resistance created by having an additional row but the potential energy gained whilst going up the lift hill would also increase so loosing more energy through the rest of the rise may not necessarily be a major issue. G force is actually a measure of acceleration (1G = 9.81m/s2) rather then force which means that it will not be a constant. The force creating the acceleration of the train will be the resultant of the mass acting down the track ( m sin θ, θ being the angle of the track from horizontal), the rolling resistance and the aerodynamic drag, all of these will increase if the mass is increased, so the resultant force will not vary a huge amount, so I think you are right in saying the acceleration will drop and the deceleration whilst rising will increase. I think it is likely that more often then not the train would still reach the end if there was an extra row, but there is a larger chance that every now and again when the conditions are right (or wrong) it won't make it. Trying to allow for this extra mass would not be a simple thing to do, you can not simply lengthen the lift hill as this would mean higher speeds at the beginning of the ride, so the size of the first loop would have to be increased and the turns would have to be opened out to allow for the increases in centripetal force to keep it within the comfortable limits for an unfit human being. All of would likely have resulted in a ride that was too big to fit in the allocated space.Another issue with increasing the weight is that it may wear the track out faster, the loading will be now where near what is required to cause an immediate failure, but it will increase the fatigue loading on the track. To stop this would have meant additional bracing and supports to keep the loading underneither the minimum loading for fatigue. This wouldn't have been difficult to do as obviously its been done, but it would cost more to do, and thorpe park may have deemed this extra cost not justifiable to get a 14% increase in throughput.I personally think that if an extra row could have been added it would have been, but a combination of extra cost and not enough space for a decent layout stopped them.
