Question:
Who was the owner of Primrose Hill Tunnel in Dec, 1929?

Context:
 Primrose Hill Tunnel is owned by British Rail from Jan, 1948 to Jan, 1994. 
 Primrose Hill Tunnel is owned by Network Rail from Jan, 2002 to Dec, 2022. 
 Primrose Hill Tunnel is owned by London and Birmingham Railway from Jan, 1837 to Jan, 1846. 
 Primrose Hill Tunnel is owned by Railtrack from Jan, 1994 to Jan, 2002. 
 Primrose Hill Tunnel is owned by London, Midland and Scottish Railway from Jan, 1923 to Jan, 1948. 
 Primrose Hill Tunnel is owned by London and North Western Railway from Jan, 1846 to Jan, 1923.

Answer:
London, Midland and Scottish Railway