HELP itinerary for train enthusiast grandson

It's doable, however I don't understand why you are staying at Shrewsbury on the nights of 26 and 27 June (it may be too late now to change that).

Especially on Sunday 28 June it is a long way from Shrewsbury to Porthmadog and back by train- Shrewsbury depart 0831, Porthmadog arrive 1148, Porthmadog depart 1800, Shrewsbury arrive 2119.

In my view you would be far better staying 2 or 3 nights somewhere on the coast. That would also allow you to include the Fairbourne Railway and the potential fun of one way to or from that railway by ferry- https://www.fairbournerailway.com/

Unfortunately the Corris Railway isn't running on your dates.

On 28 June, if you were there earlier you could add the Boston Lodge works tour at 1000. The problem is that lasts to 1225, but the ONLY train to Blaenau Ffestiniog leaves at 1040.

While the Tallyllyn works on any day, possibly the best day of the week is a Thursday (25 June in your case) when they have the Slate Trail special guided train trips.

So maybe this needs more thought. Staying in Shrewsbury, on both days, you are going straight past the Welshpool and Llanfair Railway- https://www.greatlittletrainsofwales.co.uk/
You are also omitting the Vale of Rheidol railway at Aberystwyth.

it's NOT doable!!!! Because the train times are all wrong.

There isn't a 08:00 from Shrewsbury on either date,

the ride from Machynlleth → Porthmadog takes considerably more than 30 minutes, more like 100 minutes

there is no 11:35 Porthmadog → Blaenau Ffestiniog or 13:30 return on ANY date in 2026 https://www.festrail.co.uk/pdf/summary-tt-2026.pdf

on 28th using the 08:31 from Shrewsbury he won't arrive Porthmadog until 11:48, well over one hour too late for the only train that day to Blaenau

and there is no 17:20 from Porthmadog, and the journey to Shrwewsbury is considerably longer than that allowed, kitchens will be closed for dinner by the time he gets back to Shrewsbury

It is definitely worth booking accommodation in Wales for the night of the 27th. It is an additional accommodation cost but you will save some money from dropping a return train journey from Shrewsbury.

I have no idea if it is ‘doable’ on paper or not. Assuming that it is they will be two very crowded days with little opportunity to actually enjoy what you are doing. They will also rely on everything going to plan and running on time which is not very likely. With this plan as it stands I think you are setting yourself up to fail sorry.

Thank you!!! I used chat got to generate and I’m so glad I asked. Would it be better to stay in Porthmadog for wales?

Yes, in reality Porthmadog (or just possibly Caernarfon) is where you need to stay if you want to do anything with the Ffestiniog/Welsh Highland.
After the Tallyllyn day it is an easy transfer onwards to Porthmadog on the train.

For my own liking I would sacrifice some time at Porthmadog to get the Tallyllyn in, if short of time.

There is a good bus service from Caernarfon to Porthmadog, but on a Sunday the first one is at 0905 arriving Porthmadog at 1000- so would miss the works tour, but make the BF train.

The reality of the current timetables is that if you want to cover everything there is at Porthmadog you need three days there- that is to do the full length of the Ffestiniog. the full length of the Welsh Highland and the Boston Lodge works tour on the Sunday morning, so there are still choices to be made.
(oh, and a quick extra, the Heritage Welsh Highland Railway- a different and very short line).

There is also a bus, the T22, links Caernarfon. Porthmadog and Blaenau Ffestiniog together (not from Caernarfon on a Sunday). Using that bus it MAY be possible to link one way journeys on the Welsh Highland and the Ffestiniog on the same day- I haven't checked for those dates. But if it is the timings would be pretty optimal.

It does not make sense to go all the way from Shrewsbury to Tywyn and then back again having done the Talyllyn. Lovely places to stay on the coast are Aberdyfi and Barmouth but Porthmadog is the best centre for both the Ffestiniog Railway and the Welsh Highland Railway to Caernarfon. The station for these lines is by the harbour at Porthmadog (Premier Inn is handy) but the normal rail station is at the northern end of this small town.

Portmeirion (Italian style village) is not far south of Porthmadog.https://portmeirion.wales

PS. Never rely on chat or AI to give advice!

Do NOT rely on ChatGPT for something like this - You need to check train times for both Talyllyn and Ffestiniog Railways using the timetables on their website.

Basing yourselves in Portmadog rather than Shreswbury is much more sensible. There nis a Premier Inn just across th road from Harbour station.

On the Ffestiniog Railway day, have you thought about booking a tour of Boston Lodge Works?

when I thought about this after my first reply I wondered whether some dreadful AI system had been involved in the original impossible plan.