Right now WE transports cannot be both formation transport and WE transports. It's one or the other - dedicated or independent. You have to choose whether it is going to operate under 1.7.5 or 3.1.3. It can never do both, so there is no need to determine which one overrides the other.
Any list that allows both attached and independent WE transports should have the same clarification on it as the Ork list with respect to Gunfortresses and Battlefortresses.
And no, that's not in the rules. It's a design determination that was made long ago because of the confusion and exploits allowed by attached transport carrying around a separate formation. Ork players were using fortresses to combine and split formations, leading to both lots of rule questions and a silly amount of unintended tactical flexibility.
Ginger: You're technically correct that an attached WE transport could embark a separate formation. That loophole is what led to the decision in the Ork list about Fortresses.
frogbear wrote:
1. Independant War Engine with a transport capacity, you can start a formation in these - correct
correct
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2. Is there any reason to have a War Engine as part of a formation and not remain Independant?
Yes. Attached WE transports gain all the advantages of 1.7.5 - move over units to board, embark/dismebark on subsequent moves, etc.. Obviously, it loses the independence of being its own formation and it cannot carry units from other formations, so it's a tradeoff.
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- War Engines as part of a formation cannot independantly move and must risk either a move that the rest of the formation decides to do to finish in it, or does nothing as the formation moves in to be transported by it. The War Engine cannot 'run over' troops to pick them up like a conventional AV transport
This is not correct. An attached WE Transport functions in all ways as a normal formation transport.
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3. Charging out of a War Engine. The rules seem to imply that if you are in a War Engine and you charge as one formation, you only have the 5cm disembark move after any War Engine movement. Effectively, if I do not wish for the war engine to move (merely offer support from where it is sitting), troops only get a 5cm charge. Is this correct?
Yep.
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Would it not make more sense that they charge out of a stationary War Engine at the full 15 to x cm?
Why would that make more sense? Most of the time you're cramming a lot more people out a not-much-larger opening (or set of openings). Also, if you only want the WE to support, you shouldn't have loaded. That's a change in plans. You're coordinating multiple formations on the fly, as opposed to them being permanently assigned and having presumably drilled on procedures. That's hard to do. Really, really freakin' hard.
As an example, is it easier to charge out the back and sides of a Plague Tower than it is for Marines to go charging out the front of a Land Raider on an assault ramp? Definitely not. If the SMs are only going 5cm, the cultists in the Plague Tower surely aren't getting any farther than that.