P <point id>   <line cat> <offset> [<side offset>]
L <segment id> <line cat> <start offset> <end offset> [<side offset>]

P 1 356 24.56
P 2 495 12.31
...

# extract lines from railroad map:
v.extract railroads out=myrr list=1

# join segments into polyline and reassign category numbers
v.build.polylines myrr out=myrr_pol
v.category myrr_pol out=myrailroads option=add

# zoom to an area of interest
g.region n=4928200 s=4921100 w=605600 e=613200

# show line, category, direction (to find the beginning)
d.vect myrailroads disp=shape,cat,dir lsize=12

# extract line segment from 400m to 5000m from beginning of line 1
echo "L 1 1 400 5000" | v.segment myrailroads out=myrailroads_segl
d.erase
d.vect myrailroads
d.vect myrailroads_segl col=green width=2

# set node at 5000m from beginning of line 1
echo "P 1 1 5000" | v.segment myrailroads out=myrailroads_segp
d.vect myrailroads_segp icon=basic/circle color=red fcolor=red size=5

# get points from a text file
cat mypoints.txt | v.segment myrailroads out=myrailroads_mypoints

# create parallel 1km long line segments along first 8km of track,
# offset 500m to the left of the tracks.
v.segment myrailroads out=myrailroads_segl_side << EOF
L 1 1 1000 2000 -500
L 2 1 3000 4000 -500
L 3 1 5000 6000 -500
L 4 1 7000 8000 -500
EOF
d.erase
d.vect myrailroads disp=shape,dir
d.vect -c myrailroads_segl_side width=2

# A series of points, spaced every 2km along the tracks
v.segment myrailroads out=myrailroads_pt2km << EOF
P 1 1 1000
P 2 1 3000
P 3 1 5000
P 4 1 7000
EOF
d.vect myrailroads_pt2km icon=basic/circle color=blue fcolor=blue size=5

# A series of points, spaced every 2km along the tracks, offset 500m to the right
v.segment myrailroads out=myrailroads_pt2kmO500m << EOF
P 1 1 1000 500
P 2 1 3000 500
P 3 1 5000 500
P 4 1 7000 500
EOF
d.vect myrailroads_pt2kmO500m icon=basic/circle color=aqua fcolor=aqua size=5