Origin of "You Can Sleep" slogan?

So, from this slogan I surmise a claim that travelers could get better sleep on the Water Level Route than on other Northeast-Midwest trunk lines.

Was this actually true, and if so why? The screeching of the wheels on curves? The jostling of attaching helper engines? Was this something that was truer earlier in history than later?

In the 1960s there were only three Northeast-Midwest trunk lines offering sleeper service: The New York Central's "Water Level Route,' the Pennsylvania via Philadelphia and Horse Shoe Curve and the Erie-Lackawanna.

None required helper service, except the New York Central in steam days going up Albany Hill, and none had curves sharp enough to squeal the wheels.

All three took great care to not upset their sleeping passengers when starting, stopping or working the train along its route.

The New York Central was able to make its claim about better sleep since their route didn't have to go through the mountains. The Pennsylvania had to do battle with the Allegheny Front leaving Altoona, and the Erie-Lackawanna (via the former DL&W trackage) had to deal with the Pocono Mountains. Engine noises like to echo off the hills.

It was largely a selling point to suggest a smoother ride--certainly it was flatter and straighter than the competition. I used to have a PRR timetable from the 1920's which featured a little box announcement in its summertime East-West schedules advertising--I forget the exact wording--the cool, refreshing mountain air as opposed to the muggy, sticky river valleys (no railroad mentioned). Hey, you advertise what you've got. Of course after air-conditioning came in during the 1930's there was no use trying that.