The perfect transit vehicle runs on rubber tires, driverless on dedicated ROWs made of concrete or human-driven on ordinary roads, mixing with normal road traffic, and will therefore resemble a bus more than a train. However, it will be possible to couple and also decouple several units into a trackless train, even during revenue operation. Since all axles of all vehicles are steerable, a train consisting of several units will still retain the same small turning circle as a single unit and the latter units will follow exactly the traces of the unit in front, thus allowing for narrow lanes even in sharp curves.
Rubber on concrete is superior for passenger traffic compared to steel wheels on rails. Rubber-tired wheels are more lightweight and they dont require heavy bogies, either, because they don’t need rigid axles. For railway vehicles, the sine running requires this. Light weight saves energy particularly when vehicles have to stop frequently on their way, as is the case in transit. While it is true that today braking energy can be recovered, it is not possible to recover even near 100% of it.
Steel wheels on steel rails have the advantage of lower rolling friction. However, this is essential only for slow and heavy traffic over long distances. Over short distances, as between the stops of a transit line, the energy lost to rolling friction is outweighted by the kinetic energy of the vehicle, and for fast traffic, by the energy lost to wind resistance. Therefore steel wheels of steel rails are superior only for slow freight traffic over long distances which is heavy and doesn’t stop often on its route. Another, albeit minimal advantage of steel wheels on steel rails is the possibility to use the rails as return conductor for electric power supply by overhead wire or third rail. Rubber-tired vehicles can be powered electrically, too, but they require a double-pole power supply.
Steel wheels on rails exert a contact pressure per unit area orders of magnitudes higher than rubber tires on concrete. This causes higher maintenance costs for railways. The low adhesion of steel on steel makes railway vehicles unable to ascend very steep slopes. Rubber-tired vehicles can easily ascend steep slopes, even with only few of their axles being powered, and they can accelerate and decelerate slightly faster than railway vehicles. Furthermore, while the braking distance in normal operation is determined by the need of maintaining comfort and safety of the passengers, thus limiting acceleration and deceleration to approximately 1.3 m/s^2, the emergency braking distance is much shorter for rubber-tired vehicles. When coupling trains within revenue operation, this allows for the vehicle behind to approach the standing vehicle much swifter without compromising safety as there is always the possibility of braking harder should the approaching vehicle transgress its target speed for a given distance. Furthermore, a shorter emergency braking distance allows for a train following onto an occupied stopping bay to under-run the absolute braking distance for normal operation to the vehicle in front after the latter already departed, thus allowing to cut off another 8 seconds or so from the minimal possible headway through a station without passing lane.
On stations, ideally there should be at least a passing lane beside the stopping lane in order to maximize throughput of the exclusive ROW and in order to allow for trains to skip stops. Of course, the costs of providing a passing lane may be prohibitively expensive for underground sections although it allows for shorter platform areas at low-utilized stations, if these are to be served by short train consists only. At stations where several trains should be able to stop at the same time, stopping bays can be aligned either side by side or, length-wise, one after the other. The self-steering capability of the vehicles and the absence of rails allows vehicles to overtake each other at every location along the route, without requiring switches.
The possibility for coupling and decoupling vehicles within revenue operation allows to customize the capacity of the trains to the demands over different stretches of the route, without falling back to short-turning trips, which has the drawback of both reducing frequency at short-turned sections and causing uneven loads at the common sections of the route. Furthermore it allows for different routings of units within the same train consists, thus serving several areas at the end of a common line at once with a high frequency, as, for instance, at the end of a common ROW, and even for units to decouple from one train, serving exclusively a low-utilized station on a line and eventually coupling at the succeeding train on the same line, even if there is no passing lane at the low-utilized station. Ideally, after coupling vehicles in normal operation, the passage way between them should be automatically opened to allow for passengers to distribute themselves evenly over the whole length of the train. For this, both ends of each vehicle should be equipped with a roller shutter.
Ideally, the dedicated ROW for transit vehicles is equipped with electric power supply overhead wires or rails, thus allowing for vehicles to be powered electrically. For spur lines on ordinary roads, however, an electric power supply would be too expensive, therefore vehicles should be equipped with an energy storage, too, allowing to travel over short distances without electric power supply overhead wires or rails.
Transit systems running on customized ROWs only have the limitation of being unable to serve all areas without them. Since customized ROWs are expensive, such areas are plenty. By vehicles being able to drive on ordinary roads and mixing with normal street traffic, they can exploit the higher travel speed on dedicated ROWs while still serving areas away from them. In order to allow for driving on ordinary roads mixing with normal street traffic without taking away space for a driver cabin from the interior, the vehicles are equipped with comprehensive camera surveillance at both ends to supervise street traffic, thus allowing for drivers driving them with their visual input being communicated to them from the cameras to monitors in front of them at a remote office. Remote-controlled driving with visual input from cameras can’t be deemed to be any less safe than driving a tramway, since, in case of sudden failure of the cameras or of the communication link, the vehicle can still emergency brake, albeit only within its exact virtual track and therefore being incapable of evading obstacles. But neither can a tramway which drives on rails within street traffic and is still deemed reasonably safe.
