All of the D&RG's motive power built in the 1880's was built with wrought iron boilers with lap seam construction. When the ICC boiler laws went into affect in 1911, new standards were developed concerning the maximum pressure a boiler could carry. These standards were based on current boiler design standards, but when applied to older boilers, they resulted in derating of the boiler pressure. Most all of the T-12, C-16, C-17, & C-19 class were reboilered in the period of 1912-17. The new boilers were of steel consruction and used butt-strap seams. I don't have exact dates as my records don't go that far back, but the guys at CRM told me the 346 got reboilered in 1914.
For a long time, I have assumed that the re-boilering went along with short smokeboxes and shotgun stacks, but that may not be 100% correct. In 1912, K-28's, 36's, etc. were a dream. The DRG was forced to reboiler much of its old power. the new boiler laws saw the final retirement of the remaining class 56's and class 45 ten wheelers, as it was thought they were too small to be worth the effort.
The locomotives built in the 1890's (C-18's, RGS T-19's) appearently were built with steel boilers as they did not get replacement boilers. The C-18's did have lap seams which eventually caused several of them to have thier boiler pressure cut from 160 to 145. I saw RGS 20 without a jacket, bt don't recall if it had lap seams. Some lap seam engines ran a long time without de-rating. The x-SP 2248 I ran in Ft. Worth still had its original 1896 lap seam boiler and was rated at 180 lbs. It had a re-enforcement strap applied behind the seam (which is either a blessing or a condemning factor - depanding on whose rules you look at).
In reality, the reason poor 318 is not running today, and needs a new boiler, is because its boiler is 100+ years old!
Interestingly, the reason we have the Eureka with under steam today is because Baldwin chose at the last minute to substitute steel for wrought iron in its boiler. It's still got its lap seam, but because of its lack of use (and living where there was good boiler water), she still is certified to carry her original rated pressure of 125lbs.
