The explanation is, apparently, that "the Network Rail system has been designed to facilitate contact not just to the adjoining cells, but to two cells in each direction. This provides a 'failsafe' safety factor so that the system will continue to function even if one transmitter is out of action, but the consequence of this is that taller masts are required – the proposed masts are typically 30 metres or more in height, and are to be installed at appropriate points along all main lines throughout the country." Such a degree of redundancy can only be warranted for safety-critical data communications required for automatic train control - that is, ERTMS. For driver-to-signaller voice communication, some redundancy may be required in very congested sections of track with multiple points and signals - but certainly not throughout the country, and certainly not to that degree.
mastsanity.org

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