line, track
線 centers on a long, thin continuous mark or path: a line, a route, or a track. The idea extends naturally to railway lines and drawn lines.
線 combines 糸 (thread) with 泉 (spring, source), which likely contributes the sound. The idea of a thread extended into a line or track.
Thread 糸 flows from a spring 泉 like a continuous line of water, forming a track or drawn line.
For セン, imagine a long line of thread stretching across a scene: scene -> セン, and the thread makes a track.
line; stripe; stria
route (bus, train, air, etc.); line
radio (communication)
Shinkansen; bullet train
radiation
circuit; line
one's line of sight; one's gaze; one's eyes (looking); glance; gaze; look
straight line
railway track; railway line; railroad; railway; track; line
(weather) front
(war) front
main line; trunk line
wiring
line
traffic lane; road lane
infrared rays; infrared radiation
front line; forefront
places alongside a railway line, bus route, major thoroughfare, etc.
batting line-up; batting order
curve
ray (of light); beam; light
wired; cabled
ultraviolet rays; ultraviolet radiation; UV radiation
electric line; electric cable; power cable
derailment
straight line
foreshadowing
dose (of radioactivity)
incense stick
main line; trunk line
shamisen; samisen; three-stringed Japanese lute
the front (of a battlefield); forefront
the whole (railway) line; the entire (bus) route
horizon (related to land)
phone extension; indoor wiring; inner line
white line
horizontal line
X-ray
popular front
38th parallel (that demarcates North Korea and South Korea)
fuse
parallel lines
horizon (related to bodies of water)
coastline; shoreline
wireless local area network; wireless LAN; WLAN
route map (for buses, trains, etc.)