A few additions: First, most manufacturers in Europe considered the adhesion qualities of diesel-hydraulic locomotives to be higher than that of diesel-electric locomotives with axles powered singly and uncoupled; adhesions of 30% were commonly quoted. The idea was that since weight transfer between axles was a leading cause of wheel slip, the coupling of three axles in a truck (in the case of both the Krauss-Maffei units and the ALCO units on D&RGW and SP) would eliminate the chance of a single axle slipping.
This is of course not to say that such units could not, and did not slip; on such a unit, one engine-truck combination could break loose and slip. This was clearly demonstrated during tests of two Rio Grande Krauss-Maffei units on the B&A portion of the NYC system, although in point of fact the units were greatly overloaded in terms of tonnage since the assumption was that one unit was roughly equal to two standard four axle units. With a unit weight of only about 331,000 lbs obviously there wasn't enough weight for this to be a valid comparison.
There has been much confusion over the actual horsepower rating of these units in general; in point of fact, the original Krauss-Maffei units were rated 4000 brake horsepower per unit, which was then derated or reduced by modification since the engines could not develop this output at the high altitudes experienced on the two purchasing railroads in the Western United States. (Information published at the time gives a brake horsepower rating PER ENGINE of about 1900 BHP at 2950 ft and about 1700 BHP at 7450 feet, or the altitude at Soldier Summit.) The next series of Krauss-Maffei units, the production roadswitcher type units for SP, were modified in design to allow a full 4000 brake horsepower at any altitude up to 7000 feet. Keep in mind that this is not the usual "horsepower for traction" rating of a US diesel, and that you will find ratings for the Krauss-Maffei units of either 3540 or 3600 horsepower for traction in official documents. I can only imagine that the ALCO units' published rating of 4300 HP is a brake horsepower rating. I recall reading the transcript of a lecture given to the RFOOA by ALCO's sales director in which he described the locomotives, and said clearly that some of the transmission limits had not yet been determined and that they only could be through experimentation and over-the-road testing. This meshes well with the previous description of originally under-rating, then uprating the ALCO hydraulic-drive locomotives.
As a final note, the continuous speed rating of a diesel-hydraulic wasn't or isn't necessarily lower than that of a diesel-electric in broad terms. The rated continuous speed of the first Krauss-Maffei units at 3800 brake horsepower, 3540 HP traction input to transmissions was 76,000 lbs at 10.5 MPH. The final Krauss-Maffei units were rated 80,000 lbs continuously at about 10 MPH. What is important to note here though is that on test on the NYC, the two Rio Grande units employed experienced violent wheelslips when wide open (Run 17) as speed dropped down to about 12 MPH - a speed above the continuous rated speed which was based on transmission heating primarily.
-Will Davis