travel from Lakes District (Keswick/Penrith) to Grantown-on-Spey Scotland, car or train better?

From Penrith to Aviemore, on a weekday, you would take a train at 1153 bound for Edinburgh and change at Haymarket, arriving at 1626.

There are buses Monday to Saturday from Aviemore to Grantown. One leaves at 1635 arriving at 1702, the next at 1725.

From Grantown buses run a few times a day, to Dufftown (then Elgin) also hourly to Aviemore and 2 hourly to Inverness via Carrbridge

By car it is 272 miles from Keswick to Grantown, about 4 3/4 hours without stops, so not a huge amount of difference. Personally I would stop at Abington services briefly and then at Ralia (near Newtonmore).

When I used to do West Cumbria to Greenock weekly Abington was always exactly 2 hours for me to the minute- so 90 minutes from Keswick. 75 from Penrith. If I lost time somewhere in Cumbria I always somehow made it up on the M74. So that means 2 hours 30 minutes from Abington to Ralia then 1 hour on to Grantown. The Abington to Ralia bit is a moveable target depending on traffic.

Thank you very much, isn31c, for your detailed info! What is the traffic like along that route, heavy/moderate/light, fast-moving, good/bad road signage?
Neither of us have ever driven that far in England (not me at all, I was the "navigator" not the "pilot" on our last excursion), and I would love to make the drive but am nervous about the traffic and navigation, especially going through or around big urban areas like Glasgow.

The road signage on all of the UK motorways is good. But the navigator needs to be on their ball in the Central belt of Scotland where you switch from the M74 to the M73, then the M73 to the M80 and the M80 to the M9- and don't be afraid to be aggressive with your lane discipline.

Most of the way the traffic should be light from Keswick through to Hamilton on the outskirts of Glasgow. The first bit on the A66 you can run into a lot of slow farm traffic, but use the fast dual carriageway stretches to make up time. The locals aim for 15 minutes for the 17 miles to the M6. If you don't want to go that fast just stay in the left hand lane on the dual carriageway sections and be impressed by how fast others will sweep past you.

I'm used to being on a schedule, you are on vacation.

For me from the west I have Carlisle to handle but the M6 bypasses Carlisle so you are fine.

Bothwell/Coatbridge for the M74 to M73 switch then up to the M80 switch is the bit where the traffic can be heavy, and unpredictably so, but "should" thin out on the M80/M9.

I've probably been really unlucky but at Stirling where the M9 becomes the A9 always seems to have been a bottleneck for me due to the roundabout and the traffic to the Trossachs but you never know. It could be easy for you. North of Stirling on the A9 be very alert to the average speed cameras which operate for long stretches. You've had your fun on the M6/M74 run. Now is the time to stick to or a bit below the speed limits. On the single lane stretches of the A9 those can be slow going due to the wagons/trucks and not being able to get past them due to the cameras. So just sit back and don't fret about it, and use the long dualled stretches to advantage.

If you need another break before Ralia due to traffic pull off at the House of Bruar shopping centre just north of Blair Atholl.

Again, amazingly detailed and helpful info, thank you so much! Yes, we will be on vacation, so no worries about needing to drive fast. Our problem will be with sightseeing and wanting to stop for good views. :-)
Much appreciate your knowledge and help!

My mother and I stopped at House of Bruar recently. Mum had been before but it was a first for me. They've got a really good setup for the visitor. Nice classy shopping for gifts and a good self service restaurant. I had locally produced sausages, which were great, and chips. Well worth a visit if you're driving up the A9.

House of Bruar the ladies loos are even fancier than the gents .