Communications-based train control
[8] As a result, Bombardier opened the world's first radio-based CBTC system at San Francisco airport's automated people mover (APM) in February 2003.
[9] A few months later, in June 2003, Alstom introduced the railway application of its radio technology on the Singapore North East Line.
In the modern CBTC systems the trains continuously calculate and communicate their status via radio to the wayside equipment distributed along the line.
These points are communicated to make the trains automatically and continuously adjust their speed while maintaining the safety and comfort (jerk) requirements.
Thus, the wayside equipment is able to establish protected areas, each one called Limit of Movement Authority (LMA), up to the nearest obstacle (in the figure the tail of the train in front).
Movement Authority (MA) is the permission for a train to move to a specific location within the constraints of the infrastructure and with supervision of speed.
[12] It is important to mention that the occupancy calculated in these systems must include a safety margin for location uncertainty (in yellow in the figure) added to the length of the train.
[13] In fact, CBTC is not a synonym for "driverless" or "automated trains" although it is considered as a basic enabler technology for this purpose.
There are four grades of automation available: CBTC systems allow optimal use of the railway infrastructure as well as achieving maximum capacity and minimum headway between operating trains, while maintaining the safety requirements.
[15] CBTC technology is evolving, making use of the latest techniques and components to offer more compact systems and simpler architectures.
For instance, with the advent of modern electronics it has been possible to build in redundancy so that single failures do not adversely impact operational availability.
[15] The use of new functionalities, such as automatic driving strategies or a better adaptation of the transport offer to the actual demand, allows significant energy savings reducing the power consumption.
In systems with poor line of sight or spectrum/bandwidth limitations a larger than anticipated number of transponders may be required to enhance the service.
An alternate method to improve system availability in tunnels is the use of leaky feeder cable that, while having higher initial costs (material + installation) achieves a more reliable radio link.
When CBTC is applied to systems that previously ran under complete human control with operators working on sight it may actually result in a reduction in capacity (albeit with an increase in safety).
For instance, CBTC introduction in Philly's Center City trolley tunnel resulted initially in a marked increase in travel time and corresponding decrease in capacity when compared with the unprotected manual driving.
This was the offset to finally eradicate vehicle collisions which on-sight driving cannot avoid and showcases the usual conflicts between operation and safety.
Despite the difficulty, the table below tries to summarize and reference the main radio-based CBTC systems deployed around the world as well as those ongoing projects being developed.
Besides, the table distinguishes between the implementations performed over existing and operative systems (brownfield) and those undertaken on completely new lines (Greenfield).
