Choosing a departure
As there are two operators which provide the trains on the Edinburgh to Penrith route, the recommendation is to use the National Rail booking service when looking up this journey.
By doing so you'll have an overview of the ticket times and prices of all departures, no matter which company is operating each departure, and you will also avoid any booking fees.
One company doesn't always offer the cheapest fares on this route, so it's worth making an easy comparison across all departures.
Booking Advance Tickets
If you want to travel at the cheapest possible price, look for the 'Advance tickets', though the four key things worth knowing about booking and using 'Advance' tickets for travel by TPE services are:
(1) The ticket(s) will be valid for the specific departure you select when making a booking.
(2) The ticket(s) cannot either be refunded, if you subsequently change your plans or miss the train due to any circumstances not attributable to the train operator.
(3) They can be exchanged to another departure on this route prior to the departure you have booked - but the price difference with the new ticket will need to be paid (the price will likely have risen since your original booking will need to be paid.
(4) TPE doesn't take Advance tickets off sale until 15 mins prior to departure.
Reservations
When you book an Advance ticket for a Glasgow to Penrith train journey, you will be automatically assigned a seat(s), but if you book the other types of ticket ahead, you can request a complimentary reservation - more info on this
Tickets can now be booked online up to a couple of hours prior to departure
However, if you buy Off Peak or Anytime tickets just prior to boarding at the station, a seat reservation WON'T be automatically included with your booking.
That's because reservations at stations need to be booked by the end of the previous day for morning departures and around two hours ahead for afternoon/evening departures - so also keep that in mind if you will be using a rail pass, including a Britrail or Eurail pass.
Travelling on Monday - Friday
Tickets are usually available 12 weeks ahead of the travel date.
However, when you look up a journey you may see dates further ahead on the calendar, but you'll be informed that tickets aren't yet available for those dates.
Travelling on Saturday-Sunday
Tickets are usually available 12 weeks ahead of the travel date, but maintenance work on the route is periodically undertaken at weekends and if it is scheduled on your travel date, it can affect when tickets will be released for sale.
If you're looking up a journey less than 12 weeks ahead and tickets aren't available, it can be a good indication that works will be impacting on your travel date.
It can be worth checking this on the National Rail website