To a first approximation, the passengers should not make any difference, for similar reasons to Galileo's (possibly apocryphal ) experiment at Pisa. The whole package converts potential energy (from the gravitation energy at the original start point) to kinetic energy. The velocity at any point depends only on the vertical height below the start point - the total mass cancels out.

But in the real world there are other considerations that could have a measurable effect.

First, the extra passenger mass is largely shielded from the airflow. You would expect the ride to lose less speed to air resistance because the energy from the extra mass is not dissipated so quickly. Some mechanical parts (like bearings with seals) have a fixed resistance independent of load, which would also benefit from the additional passenger weight.

Second, the passenger mass is further from the rails than the machinery mass. In a vertical loop, that affects the average radius of the mass. So you would expect the ride to go faster in an inside loop (because the linear momentum becomes angular momentum at a smaller average radius, so the angular velocity must be higher), and correspondingly you should go slower on outside loops.

Since my first answer, I remembered a transport proposal (probably from a Sci-Fi writer, but I can't remember the source). The idea is wonderful, but the engineering might be a bit challenging for a couple of centuries.

Basically, build a pair of tunnels from London to New York. These have to descend (with maybe a 10% slope) to 5 miles deep at each end, and then maintain a constant depth in between. They have to be airtight , and all the air pumped out. They have railway track through them. You need trains that have pressurised cabins and very low-friction running gear (maybe liquid-helium bearings and so on). You need air-locks to allow the trains in and out.

The connection with roller coasters is this: once you nudge the trains to the start of the slope, they need no external energy (apart from replacing rolling gear losses), they just run on the gravitational energy from the initial descent. There is no loss due to airflow in a vacuum. Further, there is no sonic boom in a vacuum. And there is no need for braking losses - the ascent at the far end just recovers the original energy as the train rises to sea level again.

How fast does such an unpowered train go? The gravitational energy is M (train mass in kg) * g (9.81 metres per sec per sec) * h (the vertical descent, around 8000 metres). And the corresponding kinetic energy is 0.5 * M * square(V) with V measured in metres/sec. I make that about 400 metres/sec, which is 900 miles per hour.

There is the original green lunch - London to New York in 4 hours with negligible energy loss.