Let me help you.
The only Narrow Gauge line actually operational in Portugal is the "Vale do Vouga" Line which makes an arch around the "Norte" line between Aveiro and Espinho, with central stop at Macinhata do Vouga.
It is a "must do" since it is bound for closure pretty soon and it's close to Oporto. But, most of all, some of its characteristics are very archaic, something like rail technologies from the 70's still working today!!!
The better way to travel in this line is a full day plan in the Aveiro -> Espinho direction with stop at the Macinhata do Vouga's railway museum and a 15min walk from Macinhata to Sernada do Vouga. You have to get from Oporto to Aveiro and from Espinho back to Oporto in commuter. It's for me the best option since it makes it possible to visit the museum and have has much time has needed to visit all the places. (<- Not sure if I should use as... as or has... has)
Shortening the information, you have this option:
- CP Porto || Suburbano || Nº 15715/4
- Departure: São Bento (8:05 AM)
- Arrival: Aveiro (9:12 AM)
- CP Regional || Regional || Nº 5107
- Departure: Aveiro (9:53 AM)
- Arrival: Macinhata do Vouga (10:48 AM)
*Visit to Macinhata do Vouga Railway Museum*
*Walk from Macinhata do Vouga to Sernada do Vouga (ask the museum conservator if it's safe to go by rail)*
*Lunch at Sernada do Vouga and visit to the place, workshops, station, etc.*
Attention! The station's bar doesn't offer that much to eat, so it's crucial to buy something and bring it from Oporto or Aveiro
- CP Regional || Regional || Nº 5211
- Departure: Sernada do Vouga (2:52 PM)
- Arrival: Espinho-Vouga (5:01 PM)
*Visit to the city for a snack. Connection to the Espinho main station*
- CP Porto || Suburbano || Nº 15641/0
- Departure: Espinho (5:56 PM)
- Arrival: Porto São Bento (6:25 PM)
If there is anything more you want to ask, more particularly about the city of Oporto itself, just ask here and I'll answer you
About photography, the Portuguese are normally very tolerant to tourists in what concerns taking photos of they're notorious landmarks (and sometimes they just feel very proud about it), but things tend to get risky when giving to much attention to technical things in stations or filming trains, or other things out of general. Sometimes the security gets to harsh (like here, at 0:20
http://www.youtube.com/watch?v=kV77LM8_D18 ) so the best thing is to always inform the security and the stationmaster about your intentions before anything.