Will they have to emulate the PRR roundhouse in nearby Crestline, and build an extra-length stall for the new "big engine" about to show up? <:-)

No, not at all. The original construction blueprint plans for the Age of Steam Roundhouse Museum were designed with both short and long stalls to accommodate different lengths of steam locomotives. Below is a portion of our hand-out that was given free to folks taking tours through The Roundhouse back in 2013. Please notice in particular paragraph #9 showing that even B&LE 2-10-4 #643 will fit into the roundhouse's current long stalls #1 through #7, as planned. Be Safe.

John B. Corns

THE AGE OF STEAM ROUNDHOUSE, TURNTABLE and BACK SHOP
213 Smokey Lane S.W. – Tuscarawas County – Sugarcreek, OH 44681
2013 / 04 / 05
For updates, information and photos, please visit our website at: http://www.ageofsteamroundhouse.org

1—The Age of Steam Roundhouse site covers 36 acres and has a 3-acre retention pond. To level-off sloping ground, 115,000 cubic yards of compacted clay fill—about 750 truckloads—were dumped to elevate the site by up to four feet. Additionally, 60,000 tons of aggregates (stone, ballast, etc.) were used.

2—The turntable pit was dug 132 feet in diameter to encase the 127-foot diameter, 2.7 million-pound concrete ring wall on which each end of the 115-foot long turntable sits and rotates.

3—Soft ground caused 178 steel I-beam pilings (5,525 lineal feet) to be driven down to bedrock under the ring wall and center pivot point to provide support for heavy locomotives rotating on the turntable.

4—For added strength, 49,000 pounds of rebar were formed inside the concrete in the turntable pit's 550 cubic yards of ring wall. Nearly 4,500 cubic yards of poured, reinforced concrete are in the AoS site.

5—Before concrete foundations could be poured for walls and tracks underneath the buildings, 1,600 treated wood pilings (each 25 feet long) were driven into the soft ground to provide frictional support for the weight of the buildings and weight of trains on the tracks.

6—Before the 1940 delivery of Western Maryland’s 4-6-6-4 Challenger-type locomotives, a 115-foot long, 400-ton capacity turntable was installed at the Hagerstown roundhouse. Moved to the Age of Steam Roundhouse in 2008, more than 57 tons of new steel was needed for repairs to this 75-year old turntable.

7—The Age of Steam turntable pit is not round. Actually, it is comprised of 196, flat, individual surfaces on the interior concrete ring wall which, when seen together, give the illusion of a perfectly circular wall.

8—Each roundhouse stall covers an angle of 6 degrees, 47 minutes and 09 seconds. Together, all 18 roundhouse stalls comprise an arc of 122 degrees, or 1/3 of a circle, and cover an area of 48,500 square feet, more than an acre (43,560 square feet). A half-mile of gutters and downspouts carry away rain water.

9—Roundhouse stalls #1 to #7 measure 127 feet long with a track length of 112 feet; stalls #8 to #18 measure 97-feet long with tracks measuring 82-feet. Roundhouse Stall #1 that leads into the back shop measures about 160 feet long.

10—Below roundhouse stalls #2, #4 and #5 are 4-foot deep, 100-foot long inspection pits. For employee comfort, all floors in the shop and in roundhouse stalls #1 through #7 are radiantly heated by pumping hot water through a spider web of tubes strategically placed in the 14-inch-thick concrete floors.

11—To reduce construction costs, ballasted tracks are used underneath short-stall, locomotive exhibit tracks #8 to #18. These 11 tracks are covered with wood planks to provide a continuously-flat floor.

12—The roundhouse is a true brick structure, with separate interior and exterior walls formed by hand-laying individual red bricks, and then filling the hollow-wall cavities with concrete. To increase or reduce wall thickness, the red bricks were set in a staggered, outward, decorative pattern called a corbel.

13—All totaled, Age of Steam roundhouse and shop required 269,825 Jumbo Colony Red bricks; 10,717 rowlock/soldier bricks; 15,377 projected headers; 25,761 concrete blocks; 286 coping tiles; and 4,750 specially-cast and fired bullnose bricks for use along the curved edges of doorway and window openings.

14—Vitrified coping tiles act like miniature peaked roofs to prevent rainwater from getting down into the tops of brick side walls having stepped parapets. These 286 specialty tiles are made of red clay with a glazed top coat, and manufactured at a kiln located in nearby Uhrichsville. The manufacturer—who still has the original molds—said that this was the first order for such coping tiles placed in the past fifty years

My question isn’t if 643 will fit… but that’s now going to be 24 steam locomotives on the property, and 18 stalls.

I imagine that at least one would be kept in the shops at all times for restoration work (as they have for #19 right now) and they have some smaller locomotives, but that’s a lot of locomotives for a roundhouse with a limited number of stalls.

Is there room for all of them in the current roundhouse?

Yep.