I fail to see how the conductor or engineer would be able to see the tanks if they were set directly underneath the bridge. It just wouldn't be within their field of view. It also looked like there was a lot of room between the bridge supports. They could have then just driven off after the heist, rather than the awkward task of removing those tanks and possibly being questioned (not that they anticipated killing the kid) by search teams.
It wouldn't be any more conspicuous than Walt was sitting off by the side with a hose stuck in the ground.
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