The "33" in the config "maxspeed" tag indicates the maximum speed of the loco in DCC/AI mode, in meters per second. 33 m/s = 73.819 MPH
I made a couple tests, one in manual DCC mode (the figure in red is a "74")
And one in IA mode:
In both cases, the ES44AC Tier 4 attained 74 MPH
In cab mode, the throttle-power container lists, for notch 8, tractive effort values equal to:
90.9 kN (20,435 lbf) @ 29.6 m/s (66.213 MPH)
0 kN (0 lbf) @ 31.3 m/s (70.016 MPH)
In the cab mode test, I managed to attain 69 MPH (the HUD speed indication is not as accurate as the one provided by instruments, as it uses an "averaged" speed: it can differ by 1-2 MPH from the actual train speed).
The test was performed on the Sherman Hill, on a 1.3% climb just west of Cheyenne, so it is possible the locomotive may indeed reach 70 MPH on a flat straight track. At that speed, however, it will be uncapable to move anything but itself for lack of tractive effort.
Note for Chris: I do not know the U.S. practices for locomotive testing: here in Italy, acceptance tests for locomotives and rolling stock include testing at 110% of their rated maximum speed (e.g. a 200 kph (125 MPH) locomotive is tested at 220 Kph (137.5 MPH)). This implies the locomotive must be able to reach at least 110% of its rated maximum speed.
Trainz Engine Editor is a fine tool, but it automatically sets the tractive effort to zero at the maximum speed you enter in the dialog box. As a consequence, the locomotive will never be able to reach that speed because it has no tractive effort. I usually specify a maximum speed value equal to 115% of the maximum rated speed, so that the locomotive has enough residual tractive effort to haul something at its maximum rated speed.