Narrow gauge was EXTREMELY common in Pennsylvania, not just the EBT.
More than half of the early logging railroads ("trams") were narrow-gauge, and the majority of Climax A's in the 13-18 ton range were narrow gauge. Regional mileage on shortlines/logging was about evenly split between standard and narrow-gauge builds.
The largest narrow gauge system surviving in the east wasn't the EBT, it was the Tionesta Valley - and it ran until 1944. 100 miles+ of track. EBT had about 60. It had passenger trains and some incredible reverse hairpin wood trestles. Colorado eat your heart out.
http://ngdiscussion.net/phorum/read.php?1,212853,213337There is a book - hard to find - "Tionesta Valley" by Walter C Casler, it's out of print and not cheap but if you want the entire story, it has been written and documented.
https://images-na.ssl-images-amazon.com/images/I/81lopq6ju9L.jpg If you look at the cover shot and see a fill made of logs, that's typical.
Some of the early oilfield railroads were narrow-gauge too; WNYP had an entire division between Bradford and Kinzua that was 3', the Warren & Farnsworth Valley was 3' with a couple beautiful 4-4-0's, and more.
I always love that Sheffield terminal shot because that's the worst mess of dual-gauge track I've ever seen.
The other big-time narrow gauge locally was the Pittsburg & Western, absorbed by the B&O and standard-gauged about 1905. This is the P&W WITHOUT the "H", deliberate. That's what became the Knox & Kane tourist railroad, now a trail. Depending on who you read, it was as long as 200 miles. It never got over its narrow-gauge heritage though if you looked at the right-of-way. Sheesh! Very little history out on it in the narrow gauge years.
In this area, the most defining statement of narrow gauge was that they avoided capital investment wherever possible. Wood instead of steel, trestles instead of fill, a hairpin curve up a valley instead of a big trestle or fill, a curve around a rock or stump. Very little roadbed. Tunnels? Are you kidding me? If there did ever turn out to be any long-term traffic, the line was standard-gauged.
The most complete 'down and out' narrow gauge locally was the Youngsville & Sugar Grove, intended as a 10-mile streetcar line but couldn't afford the wire, had a NY Elevated Forney and two cars. That's it.
Another thing of interest is that Bob Richardson, who became famous documenting the Colorado Narrow Gauge after WWII, cut his teeth here photographing the logging and narrow gauge lines of the east right before the war. His collections on these lines are now online in the Denver Public Library and of outstanding quality. After he retired from the Colorado Railroad Museum, he returned back here to live as a virtual hermit in woods of PA until he died.
I've become interested in this because the 'other' logging railroad at West Hickory was 3' - McCabe Lumber with a 3' Class A Climax. So I'm tinkering now with Nn3.
My own little Hickory Valley was also dual gauge for many years as their first locomotive was a 48" 0-4-0t that never was photographed, and the original 'Reno Railroad' branch was built to 48". The only proof of the dual-gauge era is the odd preponderance of what looks like guard rail nearly everywhere, it's the dual-gauge track that outlived the locomotive.
http://www.randgust.com/HVbridge1908.jpg