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PARANOID CLARIFICATION: This post does not refer to trains in any way. It refers to the drive system proposed by Combine Cruiser.
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I think I get what you are saying. I'm envisioning a wheel sitting in the center of a horizontal track [EDIT: By which I mean a mechanism inside the chassis itself], attached with arms.
Something like this? (Beware: painfully bad drawing and photoshop enhancement skills!)
I see where you are going with it. The entire chassis of the vehicle would be basically a reinforced armored siege cannon, but having wheels that could turn 360 degrees on their horizontal axis would still give it the maneuverability to strafe opponents.
The advantages would be a great deal of firepower.
The disadvantages however, would still be primarily a lack of mobility.
-The weapon would take power away from the drives, in all likelihood forcing the vehicle to slow down slightly before firing.
-The weapon would probably have a long recharge time.
-The vehicle could still be easily flanked by smaller faster units, and be unable to retaliate quickly.
I think it would actually prove to be a pretty balanced design, good for punching through defenses, but much less useful against a mobile enemy.
There is a vehicle in Cyberstorm 2 that I think it would end up looking very similar to. I wish I could get a screenshot, but I don't have the game with me at the moment.