“A model railroad/railway never finished” is a saying in the hobby of model railroading. There is always something to add or to remove or to modify.
   
“The section gang always needs men, for a model railway is never finished” was printed in Popular Mechanics magazine in December 1937. “A model railroad is never finished” was printed in the New York (NY) Herald Tribune on July 18, 1938. “The old adage says a model railroad is never finished, as there’s always something to add, modify, remove, replace, etc.” was posted on Twitter by Erik Paulson on October 24, 2019.
   
       
Wikipedia: Rail transport modelling
Railway modelling (UK, Australia and Ireland) or model railroading (US and Canada) is a hobby in which rail transport systems are modelled at a reduced scale.
 
The scale models include locomotives, rolling stock, streetcars, tracks, signalling and landscapes including: countryside, roads, bridges, buildings, vehicles, urban landscape, model figures, lights, and features such as rivers, hills, tunnels, and canyons.
           
Google Books
December 1937, Popular Mechanics, “Railroading for Men Only,” pg. 120A, col. 2:
The section gang always needs men, for a model railway is never finished.
 
Newspapers.com
18 July 1938, New York (NY) Herald Tribune, “Model Society Mourns Death Of Old Railroad” by John O’Reilly, pg. 11, col. 1:
A model railroad is never finished.
 
Newspapers.com
3 August 1938, Daily News (New York, NY), “Model Engineers Plan New Railway System,” Brooklyn sec., pg. 7, col. 1:
“A model railroad is never finished,” added Meredith E. Smith of 1359 Dickens Ave., Rockaway.
       
Google Books
Railroad Model Craftsman
Volume 12
1943
Pg. 67:
They say a model railroad is never finished, but mine is nearly so.
 
Google Books
Modular Model Railroading
By Jeff Griffin
Blue Ridge Summit, PA: TAB Books
1986
Pg. 120:
But, as everyone knows, a model railroad is never finished.
     
Google Groups: rec.models.railroad
HO Railroad That Grows
David Tunbo
9/3/97
(...)
I thought Linn Wescott said it best, that “a model railroad is never finished.”
 
Google Groups: uk.rec.models.rail
Copenhagen Tunnel
Mal Thomas
7/9/01
Geez….after 12 years I would have thought they would be pretty close. Still, as I have often heard, a model railway is never finished.
     
Google Groups: uk.rec.models.rail
Largest layout
Jane Sullivan
9/22/07
(...)
A model railway is never finished?
     
Twitter
MTFB
@ModelTrainHelp
A Model Railroad Is Never Finished http://post.ly/53SBh
11:46 PM · Jan 31, 2012·Posterous
 
Twitter
Dan (Dalekslayer1)
@Dalekslayer1
It’s true what they say, a model railway is never finished!
7:05 AM · May 11, 2014·Twitter for iPad
 
Twitter
Chris Sykes
@N_Amberfield
Replying to @50046Gary
As the saying goes in this hobby - a model railway is never “finished” 😊
4:33 PM · Dec 2, 2018·Twitter for iPhone
     
Twitter
Erik Paulson
@ErikPaulson1976
Replying to @TCMRM
The old adage says a model railroad is never finished, as there’s always something to add, modify, remove, replace, etc.
8:38 AM · Oct 24, 2019·Twitter for iPhone
 
The Times (London, UK)
Rod Stewart’s favourite track is his New York model railway
Ben Ellery
November 13 2019, 12:01am
(...)
An old adage states that a model railway is never finished.