hl2 additional traincar

This download looks like an addon. You need to extract the downloaded zip and put the addon folder in your Garry's Mod addons folder. You should end up with a directory structure similar to below:

• steamapps
  • common
    • garrysmod
      • garrysmod
        • addons
          • hl2 traincar <-- Addon folder!
            • e.g.
            • lua
            • maps
            • materials
            • models
            • etc

• hl2 traincar/
  • info.txt
  • materials/
    • models/
      • props_trainstation/
        • black.vmt
        • metaltrainwheel001a.vmt
        • train_001carblue.vmt
        • train_001carblue.vtf
        • train_001carend.vmt
        • train_001carend.vtf
        • train_001carroof.vmt
        • train_001carroof.vtf
        • train_001carside.vmt
        • train_001carside.vtf
        • train_001carunder.vmt
        • train_001carunder.vtf
  • models/
    • props_trainstation/
      • train001car.dx80.vtx
      • train001car.dx90.vtx
      • train001car.mdl
      • train001car.phy
      • train001car.sw.vtx
      • train001car.vvd
  • settings/
    • spawnlist/
      • moreproppertrains.txt
      • train001car.txt

dmitri, ayabriones, adrian.kropski, re15revisited, aryaputrabip, b0ssANtonIo97B:), | Ekke, UneditedMold893 #VOTE LEAVE EU, alx81, RailFan9089, MrRuck, remez334, 2205381811, Harambe©, Churrasco Diabolico., Churrasco Diabolico., justin.anderson1111, thebonniegamer37, SDoubleJC, Księciuniu, blackopsfast5, storm_angel, Combine Commander, railfan9089, bogdi11512, H1KE, muhamadalfikri_pesawat, muhamadalfikri_pesawat, uvakin.2002, Swarpeda ♥, douglasfelicio193, trancerz, Fluttershy#RO, VesxterCry, VesxterCry, notdoppler83, tesnexus.com, DeadRainBowDash9124, straightontarget, Ramirez, Illumini, GladDOS, LeoGaming_YT, LMinecrafter, BigWAT, iulian.horlescu, Legoben2, nongteam_seka, hERR WUR5T, g8001 +19 others