The answer (as has been explored before in other threads) depends on the definition of 'most powerful' in this context. I still don't know what "reactive effort" is, but tractive effort (TE) is essentially the ability to move a load without loss of adhesion/traction. Modern freight locos such as 59s, 66s, etc, have enormous TEs compared with their steam predessors. For example, a 9F had a starting TE of about 40,000lb, whereas a 59 has not far off 115,000lb. However, a 9F could probably produce 2,500hp - 3,000hp for relatively short durations, not much less than the continuous 3,300hp from a 59. The hp output indicates its ability to haul a certain load at a certain speed on a certain gradient, but does not indicate how 'sure-footed' it would be, so to speak. Large 'pacifics', such as 'West Countries' or 'Merchant Navies', for example, could produce between 2000hp and 2500hp, and thus haul a substantial train at high speed, but would not have been suitable for heavy freight work where their low to mid-30,000s lb TE would be insufficient.
By the way, don't forget that the thread is about modern scheduled motive power combinations.
