This is a (hopefully) complete list of the railway artillery used by the Wehrmacht, compiled by Antonio Pena with additional info from the other members of the forum.

German Weapons
15.0 cm Kanone (E) Introduced in 1937, too light calibre, 18 made.
17.0 cm Kanone (E) Intended to replace de 15.0 cm K, but only 6 made. The gun is the old 17.0 Schiffskanone L/40
20.3 cm Kanone (E) Guns of naval origin intended for heavy cruisers. Used as Coastal defence in France
21.0 cm Kanone (E) In service in 1938, a very long range gun (115000 m)
24.0 cm Kanone (E) Theodor An ex-naval gun, the 24.0 cm SK L/40
24.0 cm Kanone (E)Theodor Bruno Another ex-naval gun, the 24.o cm SK L/35
28.0 cm Kanone (E) kurz Bruno Made under 1936 Programme, ex-naval gun 28.0 cm SK L/40
28.0 cm Kanone (E) lange Bruno Second gun of the 1936 Programme, ex-naval gun 28.0 cm SK L/45
28.0 cm Kanone (E) schwere Bruno Third gun of the 1936 Programme, ex-coastal gun 28.0 cm KK L/42
28.0 cm Kanone (E) Bruno neue This piece was a completely new piece not a conversion of an old gun
28.0 cm Kanone 5 (E) One of the best railway guns ever made. Some 25 made
38.0 cm Kanone (E) Siegfried Converted naval 38.0 cm SK C/34 guns. 4 used as coastdefence in Norway, the other 4 converted to coast defence in France
40.6 cm Kanone (E) Adolf Also converted naval gun this one the 40.6 cm SK C/34
80.0 cm Kanone (E) Gustav Designed by Krupp and first gun completed in 1941, the second (Dora) in 1942. The Gustav Gerät used in Sevastopol in 1942

Captured Weapons
15.2 cm Haubitze (E) 455 (r) Ex russian Howitzer M 1937 on railway mounting
16.4 cm Kanone (E) 453 (f) Ex french Cannon de 164 M 93/96
16.4 cm Kanone (E) 454 (f) Ex french Cannon de 164 M 93/96
19.4 cm Kanone (E) 93 (f) Ex french Cannon de 194 M 70/93 rayee a gauche
19.4 cm Kanone (E) 486 (f) Ex french Cannon de 194 M 70/93 sur affut tous azimuts
24.0 cm Kanone (E) 556 (f) Ex french Cannon de 240 M84/17 Saint Chamond
24.0 cm Kanone (E) 557 (f) Ex french Cannon de 240 sur affut M 84
24.0 cm Kanone (E) 557/1 (f) Ex french Cannon de 240 sur affut M 17
24.0 cm Kanone (E) 558 (f) Ex french Cannon de 240 sur affut M 93/96
27.4 cm Kanone (E) 592 (f) Ex french Cannon de 274 M 17 sur affut a glisement
27.4 cm Kanone (E) 594 (f) Ex french Cannon de 274 M87/93
28.5 cm Kanone (E) 605 (f) Ex french Cannon de 285 sur affut M 17
30.5 cm Kanone (E) 636 (f) Ex french Cannon de 305 M 93/96
30.5 cm Kanone (E) 637 (f) Ex french Cannon de 305 M 06/10
32.0 cm Kanone (E) 651 (f) Ex french Cannon de 320 M 70/84
32.0 cm Kanone (E) 651/1 (f) Ex french Cannon de 320 M 70/84 with longer chamber
32.0 cm Kanone (E) 652 (f) Ex french Cannon de 320 sur affut M 17
34.0 cm Kanone (E) 673 (f) Ex french Cannon de 340 M 12
34.0 cm Kanone (E) 674 (f) Ex french Cannon de 340 M 12 rayee a 6º
34.0 cm Kanone (E) 675 (f) Ex french Cannon de 340 M 12 rayee a 4º
37.0 cm Haubitze (E) 710 (f) Ex french Howitzer de 370 M 15 Filloux
37.0 cm Haubitze (E) 711 (f) Ex french Howitzer de 370 M 15
37.0 cm Kanone (E) 714 (f) Ex french Cannon de 370 M 75/79
40.0 cm Haubitze (E) 752 (f) Ex french Howitzer de 400 M 15/16
52.0 cm Haubitze (E) 871 (f) Ex french Howitzer de 520 M 16