There's after market trading on venues. But because most of the liquidity has "gone home" you can't transact as much at those prices and those prices are based on relatively few/small transactions.
Yeah, this is where a system like this shines. In that you can build a tool, like top, but for the stock market. You have all the real-time data. So, you can look at where the party is happening at. Just by looking at the trade flows. This is where building your own system absolutely kills anything else.
