Someone on the forums wanted to allow engineers to build on his moving train. Here's an example map with this feature. Download This example only attaches dispensers and sentries. This is because teleporters will make the moving platform stutter. If you try to place a teleporter on it, it'll break instantly. However, if you want it to attach teleporters anyway, change the trigger's filter to filter_buildables_only_noattached and remove the trigger_hurt. It can sometimes stutter with dispensers and sentries as well though. It's a bit buggy but it works!