 Online retail brokerage TD Ameritrade Holding Corp said some of its thinkorswim desktop trading platform customers were unable to log on for a second-straight day, as the SP 500 .SPX hit a new all-time intraday high on Tuesday. In the meantime, clients can access their accounts through thinkorswim Web, the thinkorswim mobile app and via all other TD Ameritrade platforms. The surge also came as Main Street investors cheered the recent elimination of trading fees by most large online brokerages, including TD Ameritrade. 