"Lionel #990 Legacy Command Set #6-14295, Lionel LCS DB-9 Cable with Power Supply #6-81499, Lionel LCS Serial Converter 2 (SER2) #6-81326 and MTH TIU/TMCC Legacy 6' Connector Cable #50-1032. "
Just to be clear, if you are happy restricting yourself to control Legacy locos from a TMCC or Legacy remote, you only need the first item, or the economy version the Cab-1L and 1L command base. The other items are needed only if you wish to control the Lionel locos through the DCS system/remote/wifi/app.
Indeed, to repeat what has been widely discussed, if you buy a new Legacy loco from here on in, you don't need any of this equipment if you are content to control the loco(s) through the free Lionel app and smart device. No additional equipment needed. Or if you want a handheld, you can spend $40 on the Lionel LionChief universal remote. This operates LionChief, LionChief +, LionChief +2.0 and newer Legacy locos that have been recently catalogued. Neither Bluetooth (the Lionel app) nor the universal remote (the LionChief system) control is available in older Legacy locos, so be aware of that if you purchase "new old stock." Legacy locos with Bluetooth are labelled on the box with the Bluetooth logo and can be operated by the original TMCC system, the Legacy system, the free Lionel app and a smart device or the $40 universal LionChief remote. Seems complicated, but it's cheap and simple potentially once you get all that clear 
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