NEW YORK ( TheStreet) -- The Warren Buffett bubble in railroads has burst twice already; is it going to be punctured again before year-end?

That is the question being posed by UBS Investment Research, and which led to a downgrade on Union Pacific ( UNP), judged by UBS the most expensive stock in the rail sector.

The rail industry has outperformed the S&P 500 by 10% since Berkshire Hathaway ( BRK.A) announced its acquisition of Burlington Northern ( BNI) on Nov. 3.

The two previous Buffet investments triggered a spike in rail shares , which in turn was shortly thereafter followed by the rail sector giving back half of those gains.

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