I know that's a damn lot of work, but we Italians never liked to settle for one engine when we could have four different engines in three versions each

in "Must have" you have engines built in high number, or which played a significant role in FS history. "Secondary" are some notorious engines, but not so important. I left a lot of them out for good.
Please consider that most of the electric locomotives actually had two codes, an even number for passenger service, an odd number for cargo duty (i.e. E.632 was the passenger version of E.633, faster but with less power due to different gears setup). It could be an easy task to add both those groups, since they share the same graphics and just have different parameters.
MUST HAVE:
FS E.326
FS E.424
FS E.428
FS E.402A
FS E.402B (FS E.403 is an evolution of 402B, but it's better known with the other name)
FS E.412 (FS E.405 is a sub-group)
FS E.444
FS E.444R (an rebuilding of 444, but much different, keep them separate)
FS E.464
FS E.626
FS E.632
FS E.636
FS E.646
FS E.652
FS E.656
E.640
CTR Minuetto (both -D diesel and -E electric)
NCDP Vivalto
E.491 (a little jewel, and the protagonist of one of the most horrorific waste of money in italian railways. Basically, 25 built, none entered regular service)
ETR200 (ETR220 and 240 are identical, but have different weight)
ETR300
ETR401 (first active tilting body in the world, mind you)
ETR450
ETR460 (470 had different paintwork, 480 and 485 had different paintwork and characteristics)
ETR600 (ETR610 had different paintwork)
ETR500 1st gen
ETR500 2nd gen
ETR500 politensione (3rd gen)
Alstom Coradia CSA
TAF - trainset
TSR - trainset
ATR100 - trainset
ALe 883 - trainset
ALe 601 - trainset
ALe 801 - trainset
ALe 724 - trainset
ALe 642 - trainset
ALe 601 - trainset
ALe 582 - trainset
Tipo 1921 coach
Centoporte coach
Tipo 1933 coach
Corbellini coach
Tipo 1959 coach
Tipo Y coach
UIC-X coach
Carrozza Vicinale Piano Ribassato coach
Due Piani coach
Gran Comfort coach
MDVC coach
MDVE coach
Z1 coach
Vivalto coach
FS ALn 56 - trainset
FS ALn 556 - trainset
FS ALn 663 - trainset
FS ALn 668 - trainset
FS ALn 772 - trainset
FS ALn 773 - trainset
FS ALn 990 - trainset
FS D.445 - trainset
FS D.443 - trainset
FS D.343 (similar to FS D.345 with different stats)
FS D.341
FS Gr. 940
FS Gr. 910
FS Gr. 897
FS Gr. 875
FS Gr. 870
FS Gr. 851
FNM 220 (not many built, but what a beauty!)
FS Gr. 835
FS Gr. 750
FS Gr. 746
FS Gr. 744
FS Gr. 743
FS Gr. 741
FS Gr. 740
FS Gr. 736
FS Gr. 735
FS Gr. 730
FS Gr. 685
FS Gr. 691
FS Gr. 470
SECONDARY:
FS E.400
FS E.454 (would love having this, it was a real beauty, but only a prototype)
FNM 600
FNM 610
FNM 630
METROSTAR - trainset
ATR220 - trainset
Stadler FLIRT - trainset
FNM 620 (short version of E.632)
ALe 840 - trainset
ALe 803 - trainset
ALe 792 - trainset
ALe 790 - trainset
ALe 540 - trainset
"Traditional" coaches (dates back to early 1900s, but run up until 1990. It's a bunch of different designs joined under a single name)
Tipo 1931 coach
Tipo 1937 coach
Tipo 1946 coach
Tip 1955R coach
Eurofima coach
FS ALDn32 - trainset
FS ALn 64 - trainset
FS ALn 442 - trainset
FS ALn 873 - trainset
FS ALn 880 - trainset
FS D.461
FS D.449
FS D.442
FS D.356
FS D.342
FS D.236
FS Gr. 981 (third rail)
FS Gr. 980 (third rail)
FS Gr. 912
FS Gr. 906
FS Gr. 905
FS Gr. 904
FS Gr. 902
FS Gr. 900
FS Gr. 899
FS Gr. 898
FS Gr. 831
FS Gr. 830
FS Gr. 737
FS Gr. 688
FS Gr. 728