Starting at Newtonmore
When I was planning my Speyside Way walk, I found that it would be easier to start at Newtonmore rather than Buckie. I found that I could get an early train and be started the walk before 10am and do a full days walk to Aviemore but if I started the walk at Buckie, it meant not getting started until the afternoon due to a 3 hour bus journey between Aberdeen and Buckie. However, if you are starting at Buckie and walking to Fochabers on your first day, this will only take you 3-4 hours to complete the 10 miles.
By Train
If you choose to start at Newtonmore or Aviemore, both are well served with trains as they are on the main line from Glasgow and Edinburgh so depending where you are staying the night before, you could be ready to start in Newtonmore or Aviemore before 10am. Alternatively, get the train to Aviemore and stay the night here before starting the next day if you are travelling from further away. There is also the nightly Sleeper service that will stop at Newtonmore by special request. Getting home from Newtonmore or Aviemore by train means getting a train to Glasgow/Edinburgh to get your connecting train from there.
By Bus
Scottish Citylink Coach services from Glasgow and Edinburgh stop in Newtonmore and Aviemore every 2 hours during the day and also the return journey from Inverness if you finish your Speyside Way walk in Newtonmore or Aviemore.
Starting from Buckie
Buckie doesn't have a train station so if you intend starting the walk at Buckie, you'll need to get a train to Aberdeen and then get the Bus to Buckie, that will drop you off on the main street, not far from the start of the Speyside Way. Train services to Aberdeen are regular and the bus from Aberdeen to Buckie is about every 30 minutes but this will take 3 hours to get to Buckie from Aberdeen so don't plan a long first day unless you are staying locally.