Here is a list of all topics with brief descriptions:

Dirty	Handles the repaint of some part of the screen
Snowline functions and data structures
Sprites for rail and rail stations.
Base sprites for elrail.	Offsets via an enum are used so a complete list of absolute sprite numbers is unnecessary
Water-related sprites.
Sloped water sprites.
Sprites for the locks	There are 4 kinds of locks, each of them is 3 tiles long
Bridge type sprites.
Tubular bridges.	Tubular bridges have 3 kinds of tiles: a start tile (with only half a tube on the far side, marked _BEG a middle tile (full tunnel), marked _MID and an end tile (half a tube on the near side, marked _END
Town/house sprites.	Used in town_land.h CNST1..3 = Those are the different stages of construction The last 2 hexas correspond to the type of building it represent, if any
House 06.	These are in fact two houses for the same houseID, so V1 and V2
Stadium.
Easter egg/disaster sprites
Cursor sprite numbers.
Animated cursors.
Rows and columns in the viewport	Columns are vertical sections of the viewport that are half a tile wide
Hierarchical widgets	Hierarchical widgets, also known as nested widgets, are widgets stored in a tree
Hierarchical widget parts	To make nested widgets easier to enter, nested widget parts have been created