In conventional mode, there's no electrical reason not to reconnect a loop to the transformer. However, if you've got a loop, why not split the bus at the transformer, run it in both directions with each end as far away from the transformer as possible. This would eliminate the small effect of resistance in the bus wire to a minimum throughout the layout. Electricity takes the shortest path that it can find.
For TMCC, Legacy, and DCS, I'm not sure if there are any problems with running a loop back to the transformer. There may be issues with reflection of the digital signals. Other bus systems that carry digital information use pull-up or pull-down resistors to terminate the bus and reduce the effects of reflection.
