• MLW M420 vs. M420R

  • Discussion of products from the American Locomotive Company. A web site with current Alco 251 information can be found here: Fairbanks-Morse/Alco 251.
Discussion of products from the American Locomotive Company. A web site with current Alco 251 information can be found here: Fairbanks-Morse/Alco 251.

Moderator: Alcoman

  by csrrfan86
What is the difference between a M420 and an M420R?


  by Alcoman
The M420 R has rebuilt RS-3 trucks under it. They were only built for the P&W.

  by Nova55
There is a few minor external differences as well.

  by BrianM
You can go to http://www.srnjrr.com and go to the roster section to see pictures of both.


  by JohnR
Are the M420Rs equipped with a DC generator or a GTA17 AC traction alternator? I’m asking because there is a picture on the following URL of one of the P&W M420Rs with the hood doors around the generator section open.

http://www.rrpicturearchives.net/showPi ... ?id=414138

While not a great photo to show the rotating electrical equipment it does not appear from this photo that this particular unit is equipped with the GTA17 as normally found on M420s. The unit looks like it is equipped with a DC generator such as a GT581. As well as using trucks from RS-3s did these M420Rs also use the rotating electrical equipment from RS-3s?



  by BrianM
The M420R's are equiped with the DC Gens. The M420's have the AC's. From what I know of the M420R'snow, they did not have the RS3 rotating electrical equioment.

  by DDG
I actually have one of the m420r's on our fleet at work yes the main gen is a dc gen a gt581 all that was used from the RS-3's is the main gen and the excitor gen otherwise they are all new minus the difference in the electrical cabnet for the traction leads. from my understnading it was done to reduce cost of the unit at purchase
  by Allen Hazen
Providence and Worcester, as a new start-up as an operating railroad, seems to have thought buying modern locomotives with recycled generators was a good idea: they also go a General Electric U18B, which also had a GT581, probably from an RS-3.
  by Engineer Spike
With the DC main generator, are the transition circuits different?
  by Allen Hazen
While we're at it, could somebody give some details about just what a GTA-17 is? In the period when the M420 was built (mid 1970s), GE was using the GTA-11 on all its domestic AC-generator locomotives, from U23B (only slightly more powerful than an M420) up to U36C. I assume that the GTA-17 was a smaller (cheaper) generator that MLW requested from Canadian GE... or was it just a GTA-11 manufactured in Canada? Do you know if it was used on any other locomotive types (e.g. MLW export models)?

This has been bugging me for years, now. Obviously a U36C needs more generator capacity than a U23B, but GE seems to have felt, with regard to its own locomotive production, that the efficiencies of having a single standard generator outweighed any savings from using a smaller one on smaller-output units.
  by JohnR
Hi Allen,

I can answer some of your questions regarding the GTA-17 alternator. A while back I obtained copy of an MLW/CGE brochure, printed in 1973, that contained the print versions of presentations given by MLW and Canadian General Electric (CGE) to personnel of CP Rail on 8 February 1973 at the Chateau Champlain in Montreal.

The CGE presentation was given by a Mr. Ray Corley and in it he discussed the improved product line that they had developed and had available for the M-line locomotives. One part of the presentation was on the alternator-rectifier systems that they cataloged. At this time CGE offered three alternators covering three different horsepower ranges. They were GTA-17 for 2000 to 2700 HP; GTA-11 for 3000 to 3600 HP; and GTA-12 for 4000 to 4500 HP.

The GTA-17 was offered in four variations, A, B, C and D. The GTA-17A and GTA-17B were basically the same alternator but intended for two different RPM (HP) ratings. The GTA-17C and GTA-17D alternators were described as optional models with different stator windings to provide higher current capacity for more traction motor circuits. A table from the brochure listing various specifications of the CGE alternators is reproduced below:

GTA- | HP | (RPM) | LIMIT |
17A | 2300 | 900 | 3400A | LRC
17B | 2700 | 1050/1100 | 3600A | 2 - GE752
17C | 2700 | 1050/1100 | 5400A | 3 - GE752
17D | 2700 | 1050/1100 | 6800A | 4 - GE752
9 | 3600 | 1050/1100 | 5400A | Superceded
11B | 3600 | 1050/1100 | 5400A | 2 or 3 - GE752
11C | 3600 | 1050/1100 | 6800A | 4 - GE752
12 | 4500 | 1050/1100 | 6800A | 3 or 4 - GE752

The application notation refers to the number of GE-752 traction motor circuits that each alternator is designed to handle, where a traction motor circuit is one or more motors in series. LRC refers to the LRC passenger locomotive(?).

Dimensionally I cannot tell you how the GTA-17 differs say from the GTA-11. Neither the brochure nor the maintenance instructions I have for the GTA-17 alternator give any overall dimensional information. From photographs I've seen of the two alternators mounted to Alco/MLW 251 engines they look basically the same in diameter and length.

I know from a maintenance manual for the CN series of M420's and a schematic for the CN HR412 that the GTA-17PB1 was used in both of these locomotives. Regarding your question as to whether the GTA-17 was used in export locomotives, I have a Bombardier locomotive brochure and with few exceptions all export locomotives listed in it have AC/DC transmissions either as standard or as an option. From information I found on a now long gone web site a few years ago I know that the export MXS-627, built for Portugal in 1978 or 1979, used the GTA-17PC1 alternator. Since this is a 6 axle 2700HP locomotive, the use of the GTA-17C is consistent with the information presented in the previous table. As CGE appeared to be the preferred electrical equipment provider for MLW/Bombardier, there would be no reason to believe that they would have used anything other than some variant of the GTA-17 alternator for locomotives in the HP range that it covered from the early 70's to the 80's when Bombardier exited the locomotive market.

One other piece of GTA-17 triva and this comes from the HR412 schematic, the no-load open circuit voltage for the GTA-17PB1 in notch 8 (1050 RPM) is 1120 VDC.

Why did CGE design an alternator to cover this HP range whereas GE in the US used the GTA-11? That I don't know. The brochure did not provide any explanation as to why CGE developed the GTA-17 alternator.

On a different subject, given that you are located in Australia, would you know any technical details about the Commonwealth Engineering CE-618 locomotives? Out of the original 11 that were built there are still three in operation, one on South Spur and two with Patricks. I haven't found alot of technical information on the Internet about them though.


  by Allen Hazen
THANK YOU! That has to be one of the best and most informative replies I've had in several years of sporadically posting questions about GE locomotives and locomotive equipment! (If history of GE generators and traction motors is one of your pet interests, there are a number of old strings here, on the Railroad.net GE forum and at the GElocos Yahoo group where people have indulged my curiosity....)
Re: CE618. Sorry, I'm no longer in Australia: migrated to Canada late last year. There's a WWWeb site with Australian diesel rosters: I think findable by Googling "Loco Page." There is an Australian railfan magazine, "Motive Power," which publishes roster changes several times a month. (I'll check my not-yet-re-sorted collection to get addresses for this.) Many late Australian Alco-engined locomotives did not have GE electrical equipment: if I had to guess, I'd say the CE618 had Hitachi motors and generators, like some of the last New South Wales Alcos. I'm not sure I have any useful technical information on them, but I will look in my "library."
  by Allen Hazen
Further to the CE618 question (*)--
(1) I change my bet about the electrical equipment. The New South Wales 80-class, built at about the same time, had Mitsubishi traction alternators and traction motors, so, if asked to bet NOW, that's what I'd guess the CE618 had.

(2) The Locopage WWWebsite, which used to have very detailed write-ups of Australian diesel locomotive classes... has moved and shrunk. The current URL is
and the technical details on each class have disappeared. Apparently they are in a "downloadable database" (mentioned on the intro page), but it downloads in a format my computer doesn't know how to open.

(3) The Australian railfan magazine "Motive Power" that I mentioned... Combines an "Extra 2200 South"-analogue news and roster update with lots of color photos, a model railroading section, over the years some technical articles... I couldn't, just now, find a recent issue (my books etc have not gotten reorganized from the move yet!), but if I turn one up in the next few days I will post their address. (Australia must have more railfans per capita than North America does: it seems to support a number of railfan magazines. "Motive Power" is the one specializing on the current locomotive scene, but technical detail is only in occasional articles-- not every issue-- and I got tired of paying for pages and pages of glossy photos.)

(*) For those not in the know: the CE618 was a diesel hood unit with a 16-251 engine rated at 2400 hp (= roughly 1800 kw), built (by Commonwealth Engineering) in 1978 for the narrow (3'6") gauge lines of the Western Australian Government Railway: the last Alco-engined locomotive design introduced in Australia. So, sharing with the M420 the property of being a "post-Alco Alco."