HI Varun,
Answer is pretty simple in this case - the STB's decision against CN Rail in the pursuit of Kansas City Southern. Since the markets largely thought they were overpaying for KCS, the ruling against CN Rail is a knock against the pending acquisition. The markets are happy about that because it means CNR is unlikely to be successful in their bid for KCS and will instead focus on operational efficiencies. It even received several analysts upgrades as a result.
This is one of those cases where the markets did not like how much CNR was paying for an acquisition and since the deal is less likely to happen, the markets are happy and sending its share price higher.
Mat