Today (June 30, 2011) was the last day of service for the 1967 Tube Stock on the London Underground’s Victoria Line. The 1967 Stock had served the Victoria Line since it opened in 1968, but with the commissioning of new signalling (Invensys Rail’s DTG-R) along with the delivery of the 2009 Tube Stock, the 1967 Stock are no longer welcome on the Victoria Line. The new signalling means that the 2009 Tube Stock are the only trains which can run on the line in passenger service; the old system (used by the '67 Tube Stock) will soon be decommissioned. Unfortunately, it does not seem likely that TfL will preserve an entire train, so today's last run was probably the very last we will see of the 1967 Stock in passenger service, on any line.
At the same time, there's a very good question to be asked: with the Victoria Line having been resignalled, where would a preserved 1967 Tube Stock run? Of course, the train could theoretically run elsewhere on the Tube, but soon there won't be many other places to go. On the Jubilee Line, SelTrac is now in use across the entire length of the line; the conventional signal heads are all bagged up, and the trainstops pinned down. Resignalling of the Northern Line with SelTrac is in progress, and the Picadilly Line will follow it. The sub-surface lines are not far behind; according to news from TfL, they are to be resignalled with Bombardier CITYFLO 650. Eventually the whole of the Tube will be resignalled, and then where will preserved trains go?
Closer to home, MTA New York City Transit has started removing wayside signals from the Canarsie Line, the Siemens Trainguard MT CBTC system having proven itself in operation. Wayside signals at interlockings are being retained, but the effect is nevertheless to make it harder to run an unequipped train on the line. This isn't an issue in normal operation, of course, since both the R143 and R160 stocks are equipped for CBTC, but it would come into play if a future nostalgia train were to run on the Canarsie Line.
In short, as railways around the world are resignalled with new technology, it becomes harder and harder to run preserved trains on the lines they once inhabited. Some will argue that this isn't really much of an issue at all—that from a museum's perspective, the best way to 'preserve' a train is to maintain it as a static display, so that it is no longer subjected to the wear and tear which comes from being operated. But when it comes to trains, there are two senses of the word 'preservation'. The first view, favored by some, is the one presented already: that trains, as museum pieces, should not be taken out and operated. But for those who've spent their lives working with these trains, the best form of 'preservation' is to keep them running, so future generations can see first-hand what it used to be like to ride the subway, or the Tube, or the transit system in their city.
I can vouch for this personally; I visited the New York Transit Museum several years before I got to actually ride on one of the nostalgia trains; you can read about things like the lights going out over third-rail gaps, but that's no substitute for riding an old train and experiencing it. You can read about how conductors used to work the doors from an elevated platform between the cars (no matter how bad the weather), but that's no substitute for riding an old car and watching the conductor climb up there and work the caps and triggers.
This, in short, is why it's important to keep old trains running, and to have a place for them to run. This goes not just for today's 'old trains', like the arnines on the subway, or the 1938 Stock on the Underground, but even cars only now being retired, like the PA-1—PA-4 stock on PATH, and cars facing retirement soon, like WMATA's 1000 Series. Preserving the heritage of these transit systems means not just preserving the cars physically, but making sure they have a place to run, because, after all, that's what trains are built to do. As the operational railway evolves, transit museums which have preserved trains in working condition may find it harder to get those trains out on the mainline, but one way or another, it's got to be done.