train_service.rb

# train_service.rb

# Graph data structure is contain from, to and distance
class Graph
  attr_accessor :from, :to, :distance
  def initialize(from, to, distance)
    @from     = from
    @to       = to
    @distance = distance.to_i
  end
end

# Train Service
class TrainService
  attr_reader :rules
  def initialize(rules)
    @rules = TrainService.convert_to_graph rules
  end

  # calculate distance
  def distance(routes)
    sum_distance = 0
    routes = routes.split('-')
    routes.each_with_index do |route, index|
      break if index == routes.length - 1
      rule = @rules.select { |rule| rule.from == route && rule.to == routes[index + 1] }
      fail 'NO SUCH ROUTE' if rule.empty?
      sum_distance += rule.first.distance
    end
    sum_distance
  end

  # convert input to graph object
  def self.convert_to_graph(rules)
    rules.split(', ').map { |rule| Graph.new(rule[0], rule[1], rule[2]) }
  end
end

# example input
rules = 'AB5, BC4, CD8, DC8, DE6, AD5, CE2, EB3, AE7'
routes = 'A-E-B-C-D'

# run
train_service = TrainService.new rules
begin
  distance = train_service.distance routes
  puts distance
rescue StandardError => e
  puts e.message
end