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Hi, I'm a little confused as to how things work.  Penzance to St Erth, St Erth to Roskear and Roskear to Truro are under absolute block so trains would simply be handed off using bell codes.  East of Truro things get a little more complicated to as the handoff to Par is to a TCB section.  I have read that in the up direction Truro has to request to Par to send the trains but on the down trains are sent automatically.  How is this done?  Does this also apply to the Par/Lostwithiel handoff?  How are trains handed from Lostwithiel to Liskeard (both under TCB) and finally how does Liskeard link to Plymouth PSB?  Sorry if I'm getting a bit technical here and thanks to anyone that can help with this.

In AB as you say the signalmen communiate by bell codes and the box in advance (the one to which the train is proceeding) will unlock the section starter signal by giving Line Clear on the block instrument. The train can then proceed.

Where TCB is in use the signals are locked by the track circuits in advance. So that the signalman in the rear can pull off as soon as the relavent track circuits are clear and the train can proceed. The signalman in the rear will let the signalman in advance know the train is coming by usually by some form of Train Describer.

Depending on age and type of train describers if a box works TCB both ways on the same line the train description can be passed automatically between boxes or the signalman has to pass it on manually.

In both cases the safety of the train is maintained because the section starter can't be pulled until either Line Clear has been obtained form the box in advance under AB or the relevant track circuits are clear under TCB.

Boundaries between different types of signalling even in TCB areas can be problematic getting the interfaces right between different ages and types of equiment can be tricky and be subject ot failure. As it fails safe there is no danger to the trains but a failure  can cause delays. The Slough Old (Panel) to Slough New (IECC) interface has cause caused problems in the past particularly on the Up Main.

Exactly how Par and Truro work I'll leave to the locals.   

Penzance through to Lostwithiel pass trains using the absolute block system. There are pockets of TCB, and IB sections but the "sections" are absolute block. Lostwithiel to beyond uses TCB. Lostwithiel & Liskeard use traditional block bells to describe trains, and then Liskeard starts the Train Describer to Plymouth & yonder.

Par & Truro is complicated as there is no requirement on the Up for Par to see the tail lamp, as there is an axle counter section. This could have been adapted to TCB years ago, but remained AB for some reason.

Hope this helps. Contact me off forum if you need to.

Best wishes