In steam’s last few decades, the largest locos became too long for existing engine servicing facilities (turntables, roundhouses). Roundhouses could easily be expanded through additions to the back walls and I have seen many examples of this in both prototypes and model kits. Turntables were another matter. Obviously, the shorter turntables would have to be out of service while a larger pit was excavated and relined and a new bridge installed. My question is how the railroads dealt with this inconvenience. I can see them adding a temporary wye track for turning engines but the only access to the roundhouse servicing facilities would have been through the turntable. I suppose the locos could be run through to the next servicing facility on the line or turned without servicing and sent back to another facility. Or would it have been more typical for the railroads to simply build a new larger turntable and roundhouse in a different location.
Other ways of turning big steam - a wye (as you mentioned), a balloon track (big loop, often used to turn entire trains like passenger trains with a set order and purpose built tailend car, as well as a big turntable.
The biggest engines would not likely have been turned at the smaller facilities anyway. Not every turntable on a given railway could handle all the locos that railway might have owned.
If absolutely necessary (and possibly as part of a major overhaul) the engine could be separated from its tender to be turned. Of course the tender would also have to be turned and then reattached; but this was not common at all. Some maintenance could be performed outside as necessary, or done in the bay directly across the turntable from the inbound track(s) as you noted.
Andrew
Someplace I saw a series of pictures about how this was done. First, they excavated the new diameter of the pit, just a few tracks at a time, taking out the old pit wall and adding the new. Then they built temporary wood cribbing to carry the tracks across the gap between the new pit wall and the old bridge. This way, only a few track were out of service at any one time. Once the new pit was all done, they could bring in the cranes, knock out all the cribbing, lift out the old bridge and drop in the new one in a fairly short time.
One method (used by UP when their 4-6-6-4’s had to be turned on short turntables) was to put some kind of chock on the turntable rails that would raise the last two axles of the 4-10 centipede tender high enough to clear the fixed approach rails on the turntable aprons. I assume the water tank was pretty empty when this was done, or the load on the tender end of the turntable bridge would have been horrendous.
The NYC went the opposite way with their 4-8-4 Niagaras. The total wheelbase (engine and tender) was held to fit a 100 foot turntable, but the rear of the tender overhung the rear axle by about ten feet.
The Rutland Railroad extended the rail 2.5 feet on each end of the turntable at Bellows Falls, VT. All locomotives could then be turned on it except their mountain locomotives. The mountains, however, could be turned by going across the river to the B&M facility at North Walpole. The North Walpole facility is now used by the Green Mountain Railroad.