Can someone explain to me how the "train tactic" actually works

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Gubby Allen said:
So, on this note could someone explain to me where the mistakes are in trains which seem to be (at least to me as a novice) set up in the right manner but achieve nothing.

As I watched it - and correct me if I've read it incorrectly - but Liquigas in the Giro seemed to try the train on about 4 of 5 different stages, one rider of theirs after another driving up the mountain at great pace, shedding plenty of riders out the back, until just Silvester Szmyd & Basso were left. Szmyd would peel off exhausted leaving Basso in front and Rodriguez, Hesjedal & Scarponi would then just ride off past him each day.

Even the Schleck teams often seemed to have O'Grady & Voight doing the same and Contador, Evans were then able to capitalise.

In those cases, where are they going wrong?
1/ Then they were clearly not strong enough to pull it off
2/ Then Basso was not strong enough to finish it off