I will to call you later.
→
I will call you later.
No 'to' after will
The train is going to arrives at 8.
→
The train is going to arrive at 8.
Going to + base form, no -s
Look at those clouds! It'll rain.
→
It is going to rain.
Evidence → going to, not will
I'm seeing the doctor tomorrow.
→
I'm seeing the doctor tomorrow.
Present continuous for fixed future plans — this one is actually correct!