GCOR roads (General Code of Operating Rules)(Western roads):

GCOR has something like 200 members railroads, so I don't know if I'm gonna get em all in here, if you see one not listed, let me know.  The following list I found is dated April 1994: http://www.sdrm.org/faqs/rulebook/title.html 

Obviously, the list is not up to date.  Another list is at (dated 2000): http://gsee.sdf-us.org/signals/docs/pdf/gcor/gcor_4-2000.pdf

1) ATSF - Atchison, Topeka and Santa Fe Railway Company
2) UP - Union Pacific RR
3) CNW - Chicago and North Western Transportation Company
4) BN - Burlington Northern Railway Company
5) SP - Southern Pacific Lines
6) CP - CP Rail System Heavy Haul-US
7) AT&L Railroad Company
8) Amtrak Western Division Peninsula Commute Service
9) Arizona and California Railroad Company
10) Arizona Central Railroad Company
11) Arizona Eastern Railway Company
12) Border Pacific Railroad
13) California and Western Railroad, Inc.
14) California Northern Railroad Company
15) California Western Railroad
16) Cedar Rapids and Iowa City Railway Company
17) Cedar River Railroad
18) Central California Traction Company
19) Central Montana Rail, Inc.
20) Chicago Central and Pacific Railroad Company
21) Columbia and Cowlitz Railway
22) Council Bluffs Railway
23) Dakota, Minnesota and Eastern Railroad Corporation
24) Davenport, Rock Island and Northwestern Railway Company
25) Duluth Missabe and Iron Range Railway Company
26) Duluth, Winnipeg and Pacific Railway
27) Eastern Alabama Railway Company
28) Fox River Valley Railroad Company
29) Gateway Western Railway
30) Grainbelt Corporation
31) Grand Canyon Railway
32) Great Western Railway of Colorado
33) Great Western Railway of Oregon
34) Green Bay and Western Railroad Company
35) Hutchinson and Northern Railway Company
36) Indiana and Ohio Rail System
37) Indiana Hi-Rail Corporation
38) Iowa Interstate Railroad, Ltd.
39) Jaxport Terminal Railway
40) Kansas City Terminal Railway Company
41) Kyle Railroad Company
42) Lake Superior and Ishpeming Railroad Company
43) Lewis and Clark Railway Company
44) Little Rock and Western Railway, LP
45) Louisiana and Delta Railroad
46) McCloud River Railroad Company
47) Meridian and Bigbee Railroad
48) MG Rail, Inc.
49) Minnesota Commercial Railway Company
50) Minnesota Valley Transportation Company
51) Montana Rail Link
52) Montana Western Railway Company
53) Mount Vernon Terminal Railway, Inc.
54)  Napa Valley Railroad Company
55) North Coast Railroad
56) Northeast Illinois Regional Commuter Railroad Corporation
57) Northern Nevada Railroad Corporation
58) Otter Tail Valley Railroad Company, Inc.
59) Port of Tillamook Bay Railroad
60) Railtex Railroad Division
61) Red River Valley and Western Railroad
62) Rochester and Southern Railroad, Inc.
63) San Diego Northern Railway
64) San Diego Railroad Museum
65) San Joaquin Valley Railroad Company
66) San Pedro and South Western Railway Company
67) Sierra Railroad Company
68) South East Kansas Railroad
69) Southwestern Railroad Company, Inc.
70) Southern California Regional Rail Authority
71) St. Maries River Railroad Company
72) Texas Limited
73) Texas Gonzales and Northern Railway Company, Inc.
74) Texas Northwestern Railway Company, Inc.
75) Toledo, Peoria and Western Railway Corporation
76) Trona Railway Company
77) Tulsa-Sapulpa Union Railway Company
78) Twin Cities and Western Railroad Company
79) Utah Central Railway
80) Utah Railway Company
81) Ventura County Railway Company
82) Washington Central Railroad Company, Inc.
83) Willamette and Pacific Railroad, Inc.
84) Willamette Valley Railroad
85) Willamina and Grand Ronde Railway
86) Wisconsin and Calumet Railroad Company, Inc.
87) Wisconsin and Southern Railroad Company
88) Wisconsin Central, Ltd.
89) Wyoming Colorado Railroad Company
90) Yreka Western Railroad