We need to make sure that forum members know that the Cercanías train line is currently closed between the T4 and Chamartín. Service is expected to reopen in early October. See
this thread
Renfe is providing a free shuttle (“lanzadera”) service from T4 to the Chamartín train station.
For those who arrive in T1, the current choices to get to Chamartín are
1. Intra-airport bus from T1 to T4, then shuttle bus from T4 to Chamartín
2. Metro, which requires two changes (in Mar de Cristal and Manoteras) - not recommended at all, not only because of the cumbersome route but also because of the high concentration of pickpockets who will be looking out for jet-lagged foreigners.
3. Cab - the best solution!