Play Safe (1936 cartoon)

In the backyard, a little boy, dressed like an engineer, is reading a book about trains while playing with a toy train. He has a brief thought about what it would be like to operate a real train. At that moment, he hears a real freight train approaching. He opens the gate to see the train stop, but his dog, Rover, grabs him before he can get far. Still wanting to get a hands-on experience, the boy attaches Rover's collar to a nearby tree. Then he makes his way toward the train and climbs onboard to the boxcar at the end. Rover frantically tries to get free from the rope, fully aware of the dangers at hand. As the train starts moving, the boy ends up falling off, knocking his head against the rails and sending him into Dreamland.

Upon entering the subconsciousness, the boy finds himself in an enormous train yard packed with odd-looking steam engines. Excited, he looks around and boards a shiny, blue streamliner. Then he hops in the engine's seat, pulls the brake lever backwards, and starts fiddling with the rest of the valves and levers. The engine starts moving slowly at first, and then at rapid speed. Suddenly, the gauges on the dashboard start scolding the boy to "play safe". Frightened, the boy reaches for the levers, but they all disappear before he can grab them. With no way of stopping the train, the boy finds himself trapped on the runaway train, which passes along the tracks leading around treacherous mountains and into a cavernous tunnel. Suddenly, from out of nowhere, a red streamliner, which accelerates as fast as the blue streamliner, appears on the same track, heading in the opposite direction. The two engines blow their whistles at one another, regardless of the inevitable collision course. Both trains scream when they meet face to face.

However, the boy is still not fully awake, and Rover hears another train coming down the tracks. Rover finally manages to break free from his collar, and he rushes to the boy's rescue. As the train speedily approaches the boy laying on the tracks, Rover finally manages to outrun the locomotive. He dips his tail in some red paint and waves it like a flag, hoping to stop the oncoming train. Alas, the engineer doesn't notice, and the engine runs into Rover, sending him speeding along the tracks toward the boy. Rover grabs the boy in his teeth and pulls him out of danger just as the train speeds right past. Once the train has gone, Rover licks the boy's face to wake him up. The boy happily hugs and kisses Rover.