Why were the Roman Army so successful for there training?

They were highly disciplined, with intelligent and effective commanders, were issued good quality equipment, and were well trained in the use of their equipment and effective combat formations and manoeuvres. The Romans also made a point of learning as much as they could about their enemies prior to engaging them in combat so they could take advantages of the enemies weaknesses and take contingencies against their strengths.

Of course this was not always the case, most of the major defeats of the Roman army can be traced to failures of one or more of these factors.