There will be engineering works between June and September on the line. The schedule hasn't been published yet but it should be like this :

- Tirano - Sondrio bus

- Sondrio - Milan train

It should take 15 min more

Here is the source (in Italian) : https://valtellinamobile.it/lavori-ferrovia-milano-tirano/

Btw for the Bernina Express if you haven't paid for the seat reservation, simply sit on the first reservation-free coaches. They have pull-down windows which is better for pics.

Alp Grüm is a nice stop for scenery along the way if you'd like :)

Schedules will be available first on www.trenitalia.com.

If you have a pass, there's no need to buy tickets: your pass is your ticket.

How i can book journey from tirano to milan including this bus?

Hi, you can't book it (reservations not possible).

If you have an Interrail or Eurail pass add it to the pass in the app or paper pass and hop on the train bus. If the bus/train is missing in the app you can add it manually. 

Check timetables on trenitalia.com

If you take the Bernina Express Bus to Lugano, a reservation is needed.

For regular tickets : on trenitalia.com like usual. Tickets are valid for 4h.

For Interrail/Eurail passes : nothing to book, everything is fully included at no extra cost. Simply add the journey to the app before boarding. If the bus times aren’t implemented until then, add the journey manually.

Thanks i was skeptical that if interrail pass will be valid on bus or not.

On rail replacement busses Interrail is valid. 

On other busses not with exceptions like Bernina Express bus from Tirano to Lugano.

Ahh we are doing the opposite route to @enzhaus and I was also getting twitchy about lack of train info, I cant tell if August isn’t published yet or if there is rail replacement buses.

Varenna-Esino → Tirano → St. Moritz  (all covered by pass I believe)

There will be rail replacement buses between Sondrio and Tirano (or Colico - Tirano for 2 weeks) : engineering works take place between mid-June and mid-September.

Yes everything is covered by the pass. If you want to travel on the panorama coaches of the Bernina Express, you need to add a 26 CHF seat reservation. But this route is best done on hourly regional trains (more flexibility and pull-down windows !)