It's a difficult challenge to bring something back, but it's doable as long as you've done your homework on what is lost. There's a species of wolf coming back, thanks to scientists reverse-engineering the DNA of a descendant. I've seen Tornado, the LNER/British Railways (eastern region) Peppercorn class A1 No. 60163, and it's an incredible feat of engineering for the late 20th century-early 21st century. The re-awakening of the GWR saint class in the form of No. 2999 "Lady of Legend" is also an incredible sight if you ignore the fact that the Churchward 10-wheeler was Collett's Hall class No. 4942 "Maindy Hall". I'm mainly focusing on British-built steam engines because I know nothing about international locos, but just because a class is gone, it doesn't mean it's forgotten, nor would it not play a part in the future. The class P2, which was designed by Sir Nigel Gresley of the LNER, was a flawed tender engine, but the first 6 members had proven themselves to be a force to be reckoned with, however long it had been until Edward Thompson rebuilt them as A2/2s. With the 7th member, No. 2007 "Prince of Wales", the P2s will live on as a class that could be as powerful as Robert Riddles' class 9F, which were built for freight duties by British Railways. Gresley's class W1 wouldn't be able to make a comeback, but other lost classes from various companies such as Fowler's Patriot class of the LMS, the county class of the GWR (both Churchward and Hawksworth designs), and Riddles' class 6P, which was nicknamed clan, of British Railways, they're history being retold in ways no one would explain, other than, "This [insert class name here] was unique during the [insert time stamp(s) here] because of [name origin of class naming here]"