Hi, currently the Alishan Foreast Railway only operates up to Fenchihu, not to Alishan. If you wish to take the Alishan Forest Railway, you must go the the Chiayi station first. The only way to book the ticket is by calling the operator at (05)225-1978, preferably in Chinese. You also can buy it in the station on the same day, but mostly the ticket will sold out. If you only have 2 days in Taichung, i think your schedule will be tight (Taichung - Chiayi station - Fenchihu by Alishan Forest Railway - Alishan - Taichung) an 1 day will not be enough. I suggest you to just take the bus from Taichung to Alishan n go back to Taichung on the same say, so that you can enjoy Taichung for one day. Or you can go to Chiayi station n take Alishan Forest Railway to Fenchihu and go back to Chiayi then Taichung on the same day. Hope it helps.