Actually Margaret, I was more referring to the Acela active tilt system; which is based upon Swiss technology, if I recall correctly - Allmanna Brown Boveri (ABB), based in Zurich. Basically, as the locomotive begins to round a curve, the degree of rotation in the lead truck is sensed, and is then used to compute the amount of hydraulic tilt needed in the following cars, to partially neutralize centrifugal force. The locomotive itself is not tilted. The time between when the lead truck on the locomotive "feels" the curve, and the first occupied passenger car hitting it, is sufficient for the system to proactively respond.

This works because the maximum speeds assigned to any railroad curve are not directly related to safety. Those calculations are instead based upon placing a certain standard limit on the centrifugal force actually felt by a passenger onboard, or the amount sideways force imposed upon stacked boxes of freight in a boxcar. These practical limits were discovered empirically over time, and are based upon comfort, and maintaining the ability to walk around in a moving vehicle.

The safety calculation results in a much higher speed, usually more than double the comfort speed. Maximum safe speed is that speed (minus a sufficient margin) around a curve, where centrifugal force will overcome gravity; causing the train to turn over (actually rotating around the axis of flange/rail contact points). When rounding a curve, the center of gravity of the vehicle focuses centrifugal force in a lateral vector (to the outside), whereas gravity, imposes a force vector downward. These forces react against the outer rail gauge corner acting as a fulcrum, thus the two vectored forces become in opposition to each other.

When the roadbed is tilted (super elevated), the net balance between the two forces is altered, generally causing a higher equilibrium speed. At equilibrium speed, there are no net sideways forces at all.

Passenger trains are generally operated somewhat above equilibrium, as measured by the additional tilt needed to bring it into equilibrium at that speed. This is expressed as "cant deficiency", which in the United States, is generally limited to 3 inches for comfort.
For example, on a given curve with say, four inches of super-elevation, a passenger train may operate at a speed requiring seven inches of cant to attain equilibrium. Turnover speed will still probably be more than double that speed.

The same comfort level over the same track with the same super-elevation, can be thus be obtained safely at much higher speeds, if the vehicles themselves add the additional needed "cant" to the passenger car floor. For example, on the same given curve with the same four inches of super-elevation, a passenger train could operate at the same comfort level at a speed requiring eleven inches to attain equilibrium, if it itself added another four inches. This is entirely safe, so long as you do not get too close to the turnover speed.

This system works so well in fact, that during the initial tests on the Swiss federal Railways, zero "cant deficiency" was easily achieved, which was thought to be ideal by the Swiss design engineers. Unfortunately, this resulted in severe dizziness and vertigo in the passengers - and "hot lunches" all over the seats! Turns out that they had to re-introduce "cant deficiency" to prevent it. Apparently, the body can sense the turn, but if it cannot also sense a congruent amount of centrifugal force, it triggers the barf button.

In any event, this active tilt system allows for much tighter control of tilt, much more tilt on much bigger vehicles, than any passive mechanism ever could. Moreover it can do it it proactively, just as the curve starts, not after the fact, as with the Spanish system. Hence the ability to size its mechanics to American size single level equipment.

Can you imagine a whole train of Budd Domes (ie. CZ) with tilt. Imagine such a train taking only 2-1/2 hrs Sacramento to Reno! Or Oakland to Sacto in 55 minutes (Oak to MTZ in 30) though all those curves - and not one foot of trackwork needed. And braking distances would still be within that allowed by the existing signal system - not very much work needed there either - just better, and single level cars (maybe eve with domes!).

OPB