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Slo-Trans is a fictional engine in Stephen King's The Dark Tower series. In the novels, the engine is part of a class of technology built by the Old Ones, a technologically sophisticated people whose culture no longer exists.
Though its actual means of operation are never discussed, the engine was apparently quite versatile and its hum is frequently mentioned as being heard in places which used large amounts of power. These include castles, transport systems and the base station of the bear cyborg Shardik. It is implied that the sound of the engine was both recognizable and persistent over a distance.
Like many artifacts of the Old Ones, the Slo-Trans engine was designed by North Central Positronics, a technology company seeking to find means to replace immortal magical phenomenon with quantifiable technological substitutes, thus leading to much of the world's strife.
Use by the Monorail System
Among other things, these very powerful engines were used to rapidly transport passengers over long distances. A monorail system employing Slo-trans engines was found which ran on batteries and could run between 4 and 6 hours at full output. The engines were initially charged in its home station (or cradle), and could be charged along the route at various locations such as The Falls of the Hounds.
The only confirmed functioning monorail was Blaine the Mono (see The Waste Lands and Wizard and Glass). Since Blaine had been operating since the time of the Old Ones, he had detailed knowledge of history and the technology that powered him. He explained that Slo-Trans technology was nearly immortal, expected to last several million years. In his case, and possibly due to the degradation of the fabric of the entire world, the engines were beginning to falter after only a few thousand years (thus dating the end of his creators' civilization). His information was suspect as he was clearly deranged.