First You should read the Doc, in regards to post Code ""code blocks" in pt. 11
Have you "Viewed" the sensor in the "States" section, there you should also see the "attributes" in it's correct "list/dict" form
in your "template" it says that
eta=1
platform=4
When your done reading, and used 3-backticks, above and below your code, it will look alot nicer, and easier to read and interpret
Try this:
{{ state_attr(‘sensor.trains_trowbridge_to_bristol_temple_meads’, ‘service’)[1].eta}}
{{ state_attr(‘sensor.trains_trowbridge_to_bristol_temple_meads’, ‘service’)[4].platform}}
or just .eta .platform