I'm surprised no-one has set up a website with pictures and details of all the different types of freight carriages. I would love to see that!
For a straightforward gallery of pictures of modern types of wagon, there are a few useful websites:
https://wagons-library.weebly.com/
https://gingespotting.smugmug.com/Wagons
https://ukrailwaypics.smugmug.com/UKRailRollingstock
https://andrewwills.smugmug.com/Wagons
The letter codes seen are from the TOPS computer system to distinguish different types of wagons. The first letter is the general kind, the second letter doesn't usually mean much but sub-divides the types further, and the third letter is the brake type. Most wagons these days are air-braked so the majority of wagon types end in A. The blue car carriers are coded WIA.
A close-up view of some WIAs with the roofs raised revealing the minis inside:
https://www.flickr.com/photos/131175774@N06/51303752257/
There are a few different numbering schemes for wagons:
Air-braked series (six digits)
B-series (6 digits with a B or often DB prefix, mainly used for some older wagons that were vacuum-braked at one time)
Private-Owner series (4 or 5 digits with an owner prefix, such as VTG which is a wagon leasing company)
International series, also known as RIV, with 12 digit numbers (including the WIA wagons). More detail on the international numbering here:
https://www.ltsv.com/rd/article_detail.php?id=14#typecodes
That last link is to the LTSV website which has a lot of information on wagons. It can be a bit daunting to use, but the wagon profiles (covering a selection of types) would be one place to start:
https://www.ltsv.com/rd/profile_list.php
===========================================================
GENERAL KIND OF VEHICLE GROUP (FIRST CHARACTER OF 'CARKIND')
===========================================================
.
CODE DESCRIPTION
---- -----------
A COACHING - PASSENGER CARRYING(INCL.CATERING VEHICLES)
B BOGIE STEEL
C CA TRAFFIC BRAKE VANS IN REVENUE EARNING SERVICE;
C CB-S COVERED (BULK CARRYING)
D DIESEL ELECTRIC MULTIPLE UNIT VEHICLES AND DIESEL MECHANICAL MULTIPLE UNIT VEHICLES
E ELECTRIC MULTIPLE UNIT VEHICLES
F FLATS (INCL.FREIGHTLINER)
G H.S.T. COACHES
H HOPPERS
I FERRY WAGONS (FOREIGN OWNED)
J PRIVATELY OWNED BOGIE WAGONS (NOT TANKS OR 'SPECIALS')
K SPECIAL WAGONS, PRIVATELY OWNED
L 'EPS' EUROPEAN PASSENGER VECHICLES 'EUROSTAR'
M MINERAL, NOT HOPPERED
N NPCCS
O OPEN
P PRIVATELY OWNED, 2 & 3 AXLE, OTHER THAN TANKS
Q SERVICE DEPT. (EX COACHING)
R OPERATING VEHICLES, BARRIER ETC.
S 2-AXLE STEEL
T PRIVATELY OWNED TANKS
U 'ENS' LOCO HAULED EUROPEAN NIGNT STOCK
V VANS
W INTERNATIONAL CAR CARRIERS
Y SERVICE DEPT. (BOGIE FREIGHT)
Z SERVICE DEPT. (2-AXLE FREIGHT)
========================
VEHICLE BRAKE TYPE CODES
========================
:
A = AIR BRAKE
B = AIR BRAKE & THROUGH VACUUM PIPE
C = UNFITTED (SELF PROPELLED DCE PLANT WITH TRAIN BRAKE PIPES)
C = UNFITTED (THESE VEHICLES CAN BE TRAIN CONSISTED)
E = ELECTRO-PNEUMATIC
F = VACUUM (A.F.I. EQUIPMENT)
G = VACUUM AND THROUGH AIR PIPE (A.F.I. EQUIPMENT)
H = AIR AND VACUUM (DUAL) (A.F.I. EQUIPMENT)
O = UNFITTED
P = THROUGH VACUUM PIPE
Q = THROUGH AIR PIPE
R = THROUGH AIR AND VACUUM PIPE
V = VACUUM BRAKE
W = VACUUM BRAKE & THROUGH AIR PIPE
X = DUAL BRAKE (VACUUM & AIR)
Y = UNFITTED (SELF PROPELLED DCE PLANT WITH POWER BRAKE)
Y = UNFITTED (THESE VEHICLES CANNOT BE TRAIN CONSISTED)
An in-depth discussion of wagon classification can be found here:
https://www.ltsv.com/rd/article_detail.php?id=12#uic