Here's chart for the Tractive Effort and amps of an SD40 from Al Krug's website, specifically the ''Railroad Facts and Figures'', ''Amperage to Tractive Effort table for an SD40-2'', under the heading ''My Tractive Effort vs EMD's'' at - http://www.alkrug.vcn.com/rrfacts/amps_te.htm
*The two lowest speed values, 7 mph & 10 mph, may bit a bit high on the estimated TE account I assume the loco is putting out its full rated 3,000 Hp. But in reality the output is probably reduced a bit account of wheelslip control at those speeds.
The accompnanying article is highly recommended reading, if a bit technical.
On another page -''Tractive Effort vs Horsepower'', at http://www.alkrug.vcn.com/rrfacts/hp_te.htm - he has this to say:
HP is the Tractive Effort (pull) times the Speed.
Burn that statement into your brain. It is crucial to understanding this essay.
Horsepower is Speed.
[snip] Note that Hp is TE times speed. If the speed remains the same and the TE (pull) increases then the Hp requirement increases. If the TE remains the same and the speed increases then the Hp requirement increases. If you have a fixed maximum Hp, such as a loco has, then as speed increases the TE must come down. The product of the two must remain a constant and is directly related to the HP rating of the loco.
I haven't found anything else other than beaulieu's explanation above of how voltage interacts with speed, and hence amps and tractive effort.
Incidentally, Krug says that thesubject Parkman Hill on the former CB&Q - at 25 miles west of Sheridan, Wyoming - is a 1.25 % grade both EB and WB - see his chart of ''Major Railroad Grades'' at http://www.alkrug.vcn.com/rrfacts/grades.htm
He also seems to imply that Parkman Hill is about 24 miles long per the following, from - http://www.alkrug.vcn.com/rrfacts/hp_te.htm
One mph is kind of slow. It would take us 24 hours just to get up Parkman hill.
If that is the case, then his average speed must have been aroubnd 40 MPH - and as a result, the amperage would have dropped below the Red Zone. But I think beaulieu's data and calcs - esp. the 15.4 MPH average speed - fits this scenario much better.
EDIT: I suppose it depends on which way he was climbing the hill. At the following link is a Map, Timetable, and Profile of the Bighorn Subdividsion:
From reviewing the profile, it appears that the Westward grade is 1.25 %, from Ranchester at MP 715 to Parkman summit at MP 724 = 9 miles; going the other way, Eastward, the maximum grade appears to be 1.30 % from Aberdeen at MP 732 to Parkman summit at MP 724 = 8 miles. However, from the data in the Timetable it appears that there is about a preceding 1.0 % grade from Wyola at MP 737 to Aberdeen at MP 732 = 5 more miles, even though on the profile it is marked as only a 0.80 % grade. But with even that added, it's still only 13 miles overall - not 24 miles. [End Edit]
Thanks again for everyone's input and insights.
- Paul North.
"This Fascinating Railroad Business"
(title of 1943 book by Robert Selph Henry of the AAR)