Factorio Set Max Train Speed Command

/c
local trains = {}
local surface = game.surfaces["nauvis"]
for c in surface.get_chunks() do
  for _, loco in pairs(surface.find_entities_filtered({area={{c.x * 32, c.y * 32}, {c.x * 32 + 32, c.y * 32 + 32}}, type="locomotive"})) do
    local main_loco
    local train = loco.train
    local locos = train.locomotives
    if train.valid and train.locomotives and (#locos.front_movers > 0 or #locos.back_movers > 0) then
      main_loco = locos.front_movers and locos.front_movers[1] or locos.back_movers[1]
    end
    local id = main_loco and main_loco.unit_number
    if id then
      trains[id] = main_loco
    end
  end
end
for _, loco in pairs(trains) do
  loco.train.speed = 1.2
end