Of course, if you take the time to stop over in Como and Lugano, you will arrive too late in Locarno to enjoy it. You'll just spend the night and then head to Lauterbrunnen in the morning.
Going to Lauterbrunnen, take the Centovalli Railway to Domodossola, then the RE 1 to Brig. Suggestion: stay on the RE 1 all the way to Spiez as it is one less train change and much more scenic than the fast way through the Loetschberg Base Tunnel, It only delays your arrival in Lauterbrunnen by 30 minutes. Sit on the left hand side for the best views.
If you take the 7 am train from Tirano (current schedule, and no reason to expect a change when the summer schedule is released), you could afford to stop for a couple of hours in Lugano. There are luggage lockers there and a funicular down the the center city. Como or Varenna train stations have no lockers, so stopping is problematic with luggage.