There have been TradeStation data and order execution APIs that were never released. So currently, TradeStation does not provide APIs like Sterling Trader, Interactive Brokers and several other brokers.
For now, as a workaround, one can interface with OOEL code running in a host window. This requires that TradeStation is open with a chart or grid running a custom analysis technique. Then a DLL can interfaces with the analysis technique to complete a homegrown API. This solution can be a good one, but the extra steps of needing a chart and special analysis technique are considered too cumbersome for many.
A workaround that we have used for years involves issuing orders like the Iceberg application and getting feedback on those orders like the RealtimeRecords application (see link). Both sending orders and receiving of feedback is done through our TSorders.dll (not advertised or sold on our site). It is similar to the TSPlatAuto.dll.
An important point is that the only programmatic trading that TradeStation officially supports is through EasyLanguage and order-entry macros in the platform. Thus with other approaches, there may be little to no recourse with TradeStation if something fails like an order not being sent or feedback not being received.
With that in mind, if one of the API approaches explained above is of interest, let us know.