The P40DC was introduced in 1993, it is currently 2013. There was an interim program to uprate the engine to 4200hp, producing the P42Dc with a P32AC-DM variant for NYC service.
For the sake of accountants and the IRS, most equipment is classified as having I think either 10 or 15 year MACRS schedule, which means that it is fully depreciated after 7 or 12 years, respectively. Anything after is considered good. As for unusually short lives, the 6000hp SD90MAC-H and AC6000CW lasted really only about 6 years with the full-rated engine. For mileage, consider running 2000 miles once every 3 days, then spinning around to do it again in another 3 hours or so without fail, at least on long-distance ops.
The first passenger diesels in the US lasted about 15 years, when the later model E and F units took over everything. They had to last until the F40PH came out, with the exception of the boiler-added freight engines (see FP45, SDP35). We then ran the wheels off the F40s until the P42 came along, so it appears that although there may be some overlap, the major epochs in American passenger locomotives are about 20 years long.
Don't forget, we can retain many common parts between models, so the Blomberg trucks on today's cabbages could very well be salvaged from one of the first covered wagons. Not sure if we will be able to say the same about GE's fabricated German-designed trucks on the Genesis.