Centrol Radio system question

["Notagunzel" <notagunzel@bigfoot.com>]

Another question for any learned correspondents.

The Victorian Centrol Train to Base radio system operates as a Land Mobile
system, where the Base transmission is on a separate frequency to the Mobile
transmit/Base receive.  Each Corridor is allocated a pair of frequencies,
and when the Base transmits, all the sites on a particular frequency
transmit simulcast.

All radio traffic is normally directly between Centrol and the Train, and
Centrol can 'patch' a Driver through to a 3rd party if necessary, eg. a
metro Signalbox or the Diesel foreman at Dynon or whatever.  There is never
radio traffic from Train to Train via the Centrol system.

Therefore there is no need for the radio traffic from the Train, on the Base
receive frequency, to be retransmitted on the Base transmit frequency.  When
it is, it is 'talkthrough' operation, as found on ARTC & ASR, and the ETRB,
and the Victorian Police for that matter.

Now for the question: on the Bendigo corridor Ch. 4 (Base tx - 419.050 Mhz &
rx - 409.600 Mhz) the Uplink (Driver) side can be heard on the Downlink
(Centrol) side, but on no other channel. Why Is It So????

P.S. the drivers don't hear it, but it can be heard with a scanner, which
prompted the question.

B.