The Dl-109 and the PA-1 are always quoted as having the SAME nominal horsepower: 2000 hp per unit. It is possible that the PA-1 was more powerful (the 2000 hp for the Dl-109 may have been generous), but I don't know. The Dl-109 was a good deal heavier than the PA-1, however, so more of its power would have gone into moving the locomotive and more of the PA-1's into pulling the train behind it. Bottom line: I am prepared to believe that the PA-1 would have been somewhat more capable when it came to pulling an express passenger train, but I'm not sure and don't have any hint about quantitative comparisons.
Now, excuse for posting to this string again. The September 1981 issue of "Trains" had an article (pp. 20-33) on the I-5 by Arthur M.. Bixby, Sr. Lots of information, including some to back up things various people have said in this string.
Item: maximum indicated horsepower of the I-5 is reported as 4400. Article doesn't say what speed this was at (I'd guess maybe somewhere roughly in the 60 m.p.h. mph ballpark), but certainly at this speed an I-5 would have had more pulling power than a pair of Dl-109. As mentioned earlier, the "Merchants Limited" was perhaps the run on which the I-5 would compete best against the Dl-109: according to Bixby, there were 50 speed restrictions where the train had to slow down from the general 70 mph speed limit, but only two station stops between Boston's South Station and New Haven: lots of need for acceleration from medium speeds to high, but not very much need for starting from a standstill. Bixby writes:
"With the advent of the Alco DL109 diesels on the new Haven, these 4000 h.p. locomotives
replaced steam on Shore Line trains-- with the exception of the "Merchants Limited." ... Since
the I-5's were specifically designed for fast acceleration from slowdowns, they could outperform
the first-generation diesels, and so they remained on the crack train afterr other runs were
(He doesn't, however, give exact dates, so we still don't know how long the I-5 lasted on the "Merchants.")
Item: as delivered, the I-5 had a serious problem with "vertical pounding" when the wheels slipped at high speeds ("particularly on wet or snowy days, or in cuts and tunnels on greasy rails"). (He mentions the Atlantic Coast Line's R-1 4-8-4 as having similar problems.) The New Haven conducted tests (which included mounting a high-speed movie camera on one locomotive to record driving wheel action) and found that drivers lifted off the the rails at wheel slips over 81 mph (a speed which-- despite the nominal 70 mph speed limit-- I-5-hauled trains would frequently reach in service). (The New York Central J-3a, which "employed lightweight reciprocating parts" and were designed with driving-wheel counterbalances which compensated less for reciprocating mass, and so came closer to perfect rotational balance, didn't have noticeable wheel lift until well over 100 mph.)
Even before the tests on the locomotive, valve-pilots were installed on all ten I-5, with recording tapes analyzed on a daily basis. "As a rule, when slips were recorded over 81 mph, the 110-pound rails of the Shore Line were found to be kinked and in need of renewal." (The ACL R-1 also inflicted kinks on the rail. ... This was in the days of jointed rail, so I suppose only a few 39 foot segments of rail had to be replaced, but it's still not something you want happening on a regular basis!) Corrective steps were taken: first, a directive to engineers to be very careful to avoid wheel-slips, with particular warnings about places like the East Haven tunnel that were known for slippery rail, and then modification of the engine balance. (Correct me if I'm wrong, but I think (lead?) balance weights were placed in compartments in the driving wheels, so the modification wouldn't have required casting new driving wheel centers.)