Well, since he's only powering the internal electronics of the TIU with the ZW, perhaps the equipment on the track is safe. However, I'd simply use a fast blow fuse for the power feed to all the TIU's, for three TIU's with let's say a combined total of six AIU boxes, I'd start with around 6-7 amps for the fuse.
About 2% of all readers will buy the fuse. About 99.% who don’t read OGR even know they need a fuse. Plus they don’t even know if the ZW is in phase.