Python3, 167 bytes

def f(N,n,t):
 q,s=[(t,0)],[t]
 for t,c in q:
  if[*range(1,N+1)]==t:return c
  for i in range(N-n):
   if(T:=t[:i]+t[i:i+n][::-1]+t[i+n:])not in s:q+=[(T,c+1)];s+=[T]

Try it online!

Perl, 120 bytes

Includes +6 for -Xapi (- and space are counted too because both the -i and the $' in the code make it impossible to combine the options with -e)

Run with the input on STDIN and the platform length after the -i option, e.g.

perl -Xapi3 trains.pl <<< "1 4 5 6 3 8 7 2 9"

Prints -2 if there is no solution

trains.pl:

1until$_=$n++*$z{pack"W*",@F}||-!grep!$z{$_}++&&/.{$^I}(??{!\$z{$`.reverse($&).$'}})$/s,keys%z,pack"W*",1..@F;$_--

if N <= 9 then you can gain 5 more bytes:

1until$_=$n++*$z{join"",@F}||-!grep!$z{$_}++&&/.{$^I}(??{!\$z{$`.reverse($&).$'}})$/,keys%z,join"",1..@F;$_--