Perhaps the biggest name in model trains in the 20th Century and beyond would have to be the Lionel Corporation. Established in 1900, the company originally provided novel (at the time) electric goods to the public such as fans and lights.
In 1901 the company produced its first train, the Electric Express. Unlike later trains from Lionel, this one was originally not intended to be sold to consumers and was simply a storefront display. Eventually, the company did produce a dozen models of the Electric Express.
Lionel Corporation dissolved in 1993. But between 1901 and 1993, thousands of different trains were produced by the company. Nowadays the trains range between common and ultra rare with prices reflecting that. Many Lionel Trains sit in the hundreds of dollars range with a number cresting over $1,000. The trains are frequently brought to PBS' Antiques Roadshow and many of them have been found to be worth in the $2,000 - $5,000 range.
As for an estimate on the original Electric Express train set? If a functioning one with its original pieces showed up, the sky's really the limit. An estimate of $10,000 - $25,000 doesn't seem particularly far-fetched in my opinion.
