This section of the Merced flows along Highway 140 through several Class IV rapids and many Class IIIs. If you put in at Red Bud, you'll run the Class IV rapids Nightmare Island and Chipped Tooth without much of a warm up.
Most people put in a few miles down from Red Bud at Indian Flat so that they have a nice warm up before the next Class IV rapid, Ned's Gulch. There are several fun rapids below here with some great places to surf both rafts and kayaks. There is an alternate take-out at a foot bridge about a mile and a half below Ned's Gulch. There is a fun Class II float down to Briceburg otherwise.
The section of river below Railroad Flat is most commonly done at lower flows (below 1,000 cfs) because the rapids are fairly continuous and dangerous at the higher flows.
Mile 0: Put in at Red Bud Picnic Area
Mile 1.2: Nighmare Island (IV) has two options and should be scouted. Chipped Tooth (IV) follows almost immediately.
Mile 2.8: River access at Indian Flat (aka Cranberry Gulch).
Mile 5.6: The South Fork of the Merced enters on the left side adding some flow. There are a bunch of fun Class III rapids below here.
Mile 7.5: Ned's Gulch (IV) is one of the bigger rapids on the Merced and can be scouted from the road on the left.
Mile 9: River access at a foot bridge that crosses the river. The rapids below here are generally Class II down to Briceburg.
Mile 15: There is river access near Briceburg. This is where highway 140 leaves the Merced river, but there is a dirt road down to Railroad Flat on the right side of the river.
Mile 17: Split Rock (IV) and Corner Pocket (IV) are two big rapids that can be scouted from the right bank.
Mile 20: Railroad Flat Campground is where the dirt road ends and is a good place to take-out unless you want to continue downstream to McClure Reservoir.
Mile 22: Quarter Mile Rapid (IV+) is a big long rapid that should be scouted from the right bank.
Mile 22.6: North Fork Falls (V+) is portaged by most groups on the right side.
Mile 28: Take-out on a dirt road on the left side that comes from Bagby.