Moscovian wrote:
We really should strive for a universal tunnel rule.
Yes, yes and yes.
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I agree with frogbear that if you want to incorporate the carriage in order to have an excuse for putting models on the board, then make it something that improves the core tunnel rules, not something integral to them. As an example, if Tunnelers scatter, then Tunnelers which emerge in LoS from the carriages get some sort of "guidance" bonus - no scatter, move their plotted point, choose placement, whatever.
As far as the tunneler transports and keeping them on the board - one idea I shared with Jaldon was to make Moles and Termites real units, but Expendable.
That makes them useful even at 0cm move and limited firepower/assault. They would be on the board at least from emerging until activation. A formation that emerges and wants to hold in place can "circle the wagons" and take advantage of them (obviously, keeping the models on the board). A formation that wants to move away for tactical reasons after emerging can abandon and suffer no BMs them under formation coherency (1.7.4). The player would just lose the abandoned tunneler vehicles.
Rather than callous disregard, Expendable can be rationalized by the fact that both the transported formations and the tunneler crew know it's a one-way trip and it's highly likely they will have to leave the vehicles behind.
A Hellbore, being a WE, can be a separate formation and survive when the mounted troops leave. Or if trying to hold, it's a separate formation that has to be broken or destroyed. Either way, the Hellbore model will definitely stay on the board.