Railroaders place to shoot the shit.

Ex-USC quarterback Todd Marinovich -- who also played for the L.A. Raiders -- was arrested for possession of meth in Irvine, CA ... TMZ Sports has learned. 

We spoke with Commander Hallinan of the Irvine PD who tells us they got a call about a naked man on the Venta Spur walking trail on Friday night ... and officers eventually found Marinovich in a nearby backyard.

Marinovich was still naked when cops arrived. We're told he was cooperative. 

Cops searched the area and found a brown bag containing meth, weed and Todd's I.D.

He was arrested and cited for possession of a controlled substance, trespassing, possession of drug paraphernalia and possession of marijuana. Marinovich has since been released. 

The 47-year-old has struggled with substance abuse since his playing days -- but seemed to be on a good path back in 2011, when he said he'd been sober for 3 years and had taken up painting.