I believe a retarded is essential. The problem is O scale cars do not act like the real thing. You need enough slope to get them rolling, and once they start they accelerate fast. When they reach level track, they decelerate fast.
A member of a local club has an operational oriented layout with a functional 4-track hump yard. The hump has enough slope to get the car to the end of the yard track, but then it doesn't have enough momentum to couple to another car. If the yard track is nearly full, the car coming off the hump rams into the parked cars and sometimes derails. Most of the times it works, and it is fun. However, a retarder would really help.
