Computer power supply common ground connected to outer rail common?

I do not think you want to mix DC power supply with AC Power Supply.  G

You can tie the DC - Ground to the AC Common safely.   Several very knowledgeable members here on the forum suggested it to me, although for a different reason than your sighting. 

Personally,   I would still run at least a 14 gauge AC common back to the transformers.   I personally want use 12 gauge stranded marine wire for my main power bus, and want to ensure that the Common side of the circuit is sized the same as the AC hot side.  

In my case,  I am running 12 VDC DPDT relays to trigger my block signals off of insulated sections of outside rails on my gargraves & ross track.  By connecting a separate DC transformer to power the 12 VDC relay solenoids,  I needed/wanted the insulated rail to "trigger" all the DC relays.   

You have to ensure all your AC transformers are "in phase" with common ground, and then you can tie the Negative DC terminal to the Common safely.   I am using an old PW Lionel 0100 HO transformer as my DC power supply, so I marked both the plug and the "direction" control lever on the transformer so that I wouldn't inadvertently reverse the DC polarity.     I am currently powering over 20 x  12 VDC signal relays this way and it has been working reliably and safely for several years.  

Thanks for the input.  The transformers have been phased and my AC common will be 12 gauge wire.  Feeds to the individual track loops will be 14 gauge.   I will wait and see if anyone else has additional comments.

I have all my buildings and some accessories running on a PC power supply at 12 VDC with the common connected to the layout AC common. Even a 5 VDC relay off the 5 VDC tap uses the same. Everything works fine.

Please ensure the power supply isolates the house AC from its outputs (i.e. open circuit when tested with a multimeter) before tying the output to the rails.  Should be no problem with computer switching power supplies, but I found battery backup boxes use "autotransformers" which share a common input and output ground.

I also have a computer power supply with common connected to my AC track common.  In my case, the DC is for a signal system.  The system runs on the filtered 12 volts DC from the power supply.  No problems.

The computer power supplies are indeed TOTALLY isolated from the 110V power line, they'd never make it out of UL certification if they weren't!  Making a common ground isn't an issue, but I suspect I'd keep them separate anyway.  UL guidelines don't prohibit an output of a power supply from being connected to the 3rd wire earth ground, that would be really bad for command operation!