Actually, even dieselized railroads can use turntables. They're very handy for cab units, once one no longer runs them in A(B)A sets. Plus, if you have a working roundhouse, it makes a fine diesel shop, and keeping the turntable is easier than laying enough turnouts to replace it. That's the main reason Pasco still had one. The old NP roundhouse handled routine diesel servicing and light repairs. And, if one wants a hood unit to face a certain direction, for whatever reason, it's cheaper, and takes less land, than building a wye.
As long as the turntable is there, works, and is paid for, there's no point taking it out.
As for two opposing ramps, for a small facility, how about a double-ended siding? Cut the track in the middle, put the two ramps in back to back, and leave enough room between them for the trucks to maneuver. You'll probably only have one or two cars at a time. On a model railroad, it might seem like a lot of space, but for the prototype it's not much room at all.
On a model, since we usually have more depth than length, I'd run two spurs facing in opposite directions, overlapped. That way the ramp for each would be near the turnout for the other, and it wouldn't take much more length than a single track. The inside ramp would have to be positioned to avoid fouling the outside track's entry, but the outside track could be as long as space allows. Pave or gravel the outside track so that trucks could get to the inside ramp, put a road, and maybe parking, on the far side of the outside track, and it's done.
In N scale, for a yard handling a couple cars at a time, two car lengths on each side would be enough for a normal day. If you have more cars, and they all come in facing one way, the switch crew will have some extra work to do. Doing it that way the entire yard could probably fit in three or four feet of length, and if you left out the parking, four inches of depth would probably work, allowing for a basic access road. Put it at the front of the layout, and parking could be off the layout, to the front, implied by the road.
